There are cases when the filename doesn’t have the right extension as it was intentionally changed either to confuse someone or of security reasons. Whenever this happens, the file can not be accessed by the application it was created with. An example is when a Microsoft Word file extension is being changed from .doc to .exe. The Operating System will automatically send an error message when the file is being launched.

In case you are using Windows Operating System, the change of a file extension is very easy however before doing so; you have to check if the system is showing the extensions of the files. If not, then you must enable the Operating System to show extensions of the files. After this you will right click on the file, select Rename then delete the actual file extension and type the new one.

In case you are using Linux or UNIX, the command for changing file extension is as follows:
mv filename.exe filename.doc

Renaming multiple file extensions
There are cases when you need to rename the file extension of many files with the same extension. For this the only thing you have to do is to type the following command in MS-DOS under the directory in which your files are being stored:
ren *.doc *.txt
The above example will rename .doc file extensions to .txt of all the files in the current directory.