How to change multiple files extensions

Renaming a batch of files can be done using a program that is specialised for that or you can just use a short command in DOS mode of your Windows. The code is not problematic, and you will be able to use it even if you are not familiar with the DOS environment.

You can get into the DOS command prompt if you go to Start -> Run. Now you need to run the "cmd" program. So just type cmd and click OK. A new window will open, that will show you in what directory you are in. You need to navigate to the directory where you have the files that you would like to rename. Use the "cd.." and "cd target-directory" commands. "cd.." will get you to a directory lower, and "cd target-directory" will get you one directory up, into the target-directory that you specify. You can always go only one directory up, so if you would like to go to c:\program files\windows, you need to do "cd program files" and then "cd windows".

Now that you are in the target directory, use the command "ren *.txt *.doc". "Ren" gives the computer the command to rename files. The second part of the command (*.txt) tells the shell to take all the files with the extension .txt and the third part (*.doc) tells it to rename all the files into the .doc extension.

If you do not want to type so much, you can use one of the many programs, that are free to use and will do the job for you. You can use, for instance the "EF Multi file renamer 1.00", free software that can rename multiple files, a whole directory and its sub directories, if you wish to do so. There are of course other programs out there, a lot of them have other functions, but you will need to pay for them, if you would like to use them.
If you are renaming files on a regular basis, then you maybe should buy one of the programs, but if you just do it a couple of times a year, then just use the DOS command, as described above. It is the old school style, but just because of that it is a reliable way to get the job done.