How to change connectionstrings that are readonly in app.config

sorry mate, posted an old one: The method GetAppSettings is just returning the config file in a dataset which is not needed.

using System;
using System.Xml;  
using System.Configuration;
using System.Collections;
using System.Reflection;  
using System.Diagnostics ;
using System.Data;

namespace BDS.Core.Config
{

      public enum ConfigFileType
      {
            WebConfig ,
            AppConfig
      }

      public class AppConfig : System.Configuration.AppSettingsReader
      {
            public string  docName = String.Empty;
            private  XmlNode node=null;

            private int _configType;
            public   int ConfigType
            {
                  get
                  {
                        return _configType;
                  }
                  set
                  {
                        _configType=value;
                  }
            }

            public DataSet GetAppSettings()
            {
                  XmlDocument cfgDoc = new XmlDocument();
                  LoadConfigDoc(cfgDoc);

                  // store it in a dataset
                  DataSet ds = new DataSet();

                  // create the dataset
                  DataTable table = ds.Tables.Add();
                  table.Columns.Add("Key");
                  table.Columns.Add("Value");

                  // retrieve the appSettings node
                  node =  cfgDoc.SelectSingleNode("//appSettings");

                  foreach(XmlElement elem in node.ChildNodes)
                  {                        
                        string[] data = {elem.Attributes[0].Value,elem.Attributes[1].Value};
                        table.Rows.Add(data);
                  }

                  return ds;
            }

            public bool SetValue(string key, string value)
            {
                  XmlDocument cfgDoc = new XmlDocument();
                  LoadConfigDoc(cfgDoc);
                  // retrieve the appSettings node
                  node =  cfgDoc.SelectSingleNode("//appSettings");
   
                  if( node == null )
                  {
                        throw new System.InvalidOperationException( "appSettings section not found");
                  }
   
                  try
                  {
                        // XPath select setting "add" element that contains this key    
                        XmlElement addElem= (XmlElement)node.SelectSingleNode("//add[@key='" +key +"']") ;
                        if (addElem!=null)
                        {
                              addElem.SetAttribute("value",value);    
                        }
                              // not found, so we need to add the element, key and value
                        else
                        {
                              XmlElement entry = cfgDoc.CreateElement("add");
                              entry.SetAttribute("key",key);
                              entry.SetAttribute("value",value);
                              node.AppendChild(entry);    
                        }
                        //save it
                        SaveConfigDoc(cfgDoc,docName);
                        return true;
                  }
                  catch
                  {
                        return false;
                  }
            }

            public bool CreateValue(string key, string value)
            {
                  XmlDocument cfgDoc = new XmlDocument();
                  LoadConfigDoc(cfgDoc);
                  // retrieve the appSettings node
                  node =  cfgDoc.SelectSingleNode("//appSettings");
   
                  if( node == null )
                  {
                        throw new System.InvalidOperationException( "appSettings section not found");
                  }
   
                  try
                  {
                        // XPath select setting "add" element that contains this key    
                        XmlElement addElem= (XmlElement)node.SelectSingleNode("//add[@key='" +key +"']") ;
                        // not found, so we need to add the element, key and value
                        if (addElem==null)
                        {
                              XmlElement entry = cfgDoc.CreateElement("add");
                              entry.SetAttribute("key",key);
                              entry.SetAttribute("value",value);
                              node.AppendChild(entry);    
                        }
                        //save it
                        SaveConfigDoc(cfgDoc,docName);
                        return true;
                  }
                  catch
                  {
                        return false;
                  }
            }
 
            private void SaveConfigDoc(XmlDocument cfgDoc,string cfgDocPath)
            {
                  try
                  {    
                        XmlTextWriter writer = new XmlTextWriter( cfgDocPath , null );
                        writer.Formatting = Formatting.Indented;    
                        cfgDoc.WriteTo( writer );    
                        writer.Flush();
                        writer.Close();  
                        return;
                  }
                  catch
                  {
                        throw;
                  }
            }
 
            public bool RemoveElement ( string elementKey)
            {
                  try
                  {
                        XmlDocument cfgDoc = new XmlDocument();
                        LoadConfigDoc(cfgDoc);
                        // retrieve the appSettings node
                        node =  cfgDoc.SelectSingleNode("//appSettings");  
                        if( node == null )
                        {
                              throw new System.InvalidOperationException( "appSettings section not found");
                        }  
                        // XPath select setting "add" element that contains this key to remove  
                        node.RemoveChild( node.SelectSingleNode("//add[@key='" +elementKey +"']") );
   
                        SaveConfigDoc(cfgDoc,docName);
                        return true;
                  }
                  catch
                  {
                        return false;
                  }
            }


            private XmlDocument LoadConfigDoc( XmlDocument cfgDoc )
            {
                  // load the config file
                  if(  Convert.ToInt32(ConfigType)==Convert.ToInt32(ConfigFileType.AppConfig))
                  {
   
                        docName= ((Assembly.GetEntryAssembly()).GetName()).Name;
                        docName +=   ".exe.config";
                  }
                  else
                  {
                        docName=System.Web.HttpContext.Current.Server.MapPath("web.config");
                  }
                  cfgDoc.Load( docName );
                  return cfgDoc;
            }

      }
}

// this should work

AppConfig ac = new AppConfig();
ac.ConfigType = (int)ConfigFileType.AppConfig;

// create a new key
ac.CreateValue("MyKey","This is the value");
// set the value of the ConnString key
ac.SetValue("ConnString","this is my connection string");