Windows operating systems have the MS-DOS command prompt that allows you to manipulate—copy, delete and rename—files. In Linux, such commands are available via a terminal window. The way to change the file extension in both Windows and Linux is to rename a file providing the new extension.
In Windows XP/Vista
In Windows XP/Vista, click "Start." Click "Run" (only Windows XP). Then type "cmd" and press "Enter" to open the command prompt window.
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In the MS-DOS shell window, type "cd c:\path\to\file" and press "Enter." Example: cd c:\user\test. If the designated file is on a different logical drive (e.g., on drive D) type "d:" and press "Enter." Then type the "cd" command as above.
Type "dir" followed by "Enter" to list the directory content and find the designated file.
Type "rename filename.ext filename.new" and press "Enter"; ".new" is the new file extension ("filename.ext" stands for any file you might have; for example: rename report.txt report.doc).
Right-click on Desktop and choose "open terminal" from the pop-up menu.
Type "cd /path/to/the/file" followed by pressing "Enter" to navigate to the directory having the file. Example: cd /home/tom/doc.
Type "ls .ext" and press "Enter" to list all files with the extension "ext," and confirm the file exists in this directory. For example: ls .txt.
Type "mv filename.ext filename.new" and press "Enter" to change the extension. For example, mv report.txt report.doc.