First method – window base
1.Open control panel and click classic view ( left side of control panel )
2.Open folder options
3.Click view tab and go to advanced settings in centre )
4.Unselect Hide extensions for known file types check box

5.Apply and ok, Now check your file name you can easily change your file extension.

Second method – Dos / command prompt

1.Open command prompt (press window logo + R and type Cmd in Run window and enter)
2.Go to directory where your file presents.
You can understand with this example, suppose your file is present in D drive and its name myfile.doc ,  if you change extension  .Doc  into .Jpg use this command .
D:\>rename myfile.doc myfile.jpg and enter

Note: 1.you can use any extension as your wish (jpg, mpeg, doc, dat , exe, and others)
2.you can also use this same method in Window xp.

Tags: Windows Vista