Change Extension
Windows makes it surprisingly difficult to find out a file's extension, and it's even more work to change it. The Change Extension tool makes light work of it just right-click a file and you'll see the extension displayed right on the context menu.
To change the file extension, click the menu item, type a new extension (with or without a leading dot), and press Enter.
To change the extensions of multiple files at once, use the Batch Rename tool.