As Will said. To see all file extensions in Windows Explorer:
On the Tools menu in My Computer or Windows Explorer, click Folder Options.
Click the View tab.
Clear the Hide extensions for known file types check box.
After you have highlighted the file to rename, click 'End', then change the
extension. This can also be achieved via a command prompt using the 'ren'
command. From a command prompt type the following to see the smitches
available: