replace an element in xml with C#

I am trying to replace an element in an xml file.  I have a solution, but it seems way to ugly.  I am asking if there is a more elegant way to solve this problem.

1. the xml file.  Here is a simplified segment of the file to show the structure
<params>
  <param>
    <name>Sample Tray</name>
    <default>Tray01</default>
    <units>each</units>
  </param>
  <param>
    <name>Dest Tray</name>
    <default>Tray02</default>
    <units>each</units>
  </param>
<params>

I want to replace the default value for one of the trays (Sample Tray for this example)
Here is what I did:
XmlDocument doc  (contains the xml file above)
XmlNode oldElement;
XmlElement root = doc.DocumentElement;
string title= "Sample Tray";
string newelementvalue = "TRAYXX";
oldElement = root.SelectSingleNode("/params/param[name='"+title+"']");
string str = oldElement.InnerXml;
int start = str.IndexOf("<default>");
int end = str.IndexOf("</default>");
string first = str.Substring(0, start + 8+1);
string last = str.Substring(end);
XmlElement newElement= doc.CreateElement("param");
newElement.InnerXml = string.Concat(first,newelementvalue,last);
root.ReplaceChild(newElement,oldElement);
doc.Save(filename);

Any Suggestions?
Thanks!!