how to rename already existing directoryinfo

Use the MoveTo memeber function
See the example, Just specify the path of the parent directory of the place where you want it to moveto.

using System;
using System.IO;

public class MoveToTest
{
    public static void Main()
    {

        // Make a reference to a directory.
        DirectoryInfo di = new DirectoryInfo("TempDir");

        // Create the directory only if it does not already exist.
        if (di.Exists == false)
            di.Create();

        // Create a subdirectory in the directory just created.
        DirectoryInfo dis = di.CreateSubdirectory("SubDir");

        // Move the main directory. Note that the contents move with the directory.
        if (Directory.Exists("NewTempDir") == false)
            di.MoveTo("NewTempDir");

    }
}