Rename the file using C# is not straight forward, but with simple trick we can achieve this. .NET framework provides File.Move() method to move file from one location to another using this method we can rename the file.
C# code sample
System.IO.File.Move(@"C:\FromFileName.txt", @"C:\TOFileName.txt");
in Vb.NET code example
System.IO.File.Move("C:\FromFileName.txt", "C:\TOFileName.txt")