Use
String.Replace
.
http://msdn.microsoft.com/en-us/library/system.string.replace.aspx[
^]
[Update]
---------
In response to your comment asking for sample code, here's the coede snippet from the MSDN documentation I linked to above:
Imports System
Public Class ReplaceTest
Public Shared Sub Main()
Dim errString As String = "This docment uses 3 other docments to docment the docmentation"
Console.WriteLine("The original string is:{0}'{1}'{0}", Environment.NewLine, errString)
Dim correctString As String = errString.Replace("docment", "document")
Console.WriteLine("After correcting the string, the result is:{0}'{1}'", Environment.NewLine, correctString)
End Sub
End Class