How to Change a Video File Extensionby Stephen Lilley
You cannot simply change the file extension of a video computer file by renaming it. This would be pointless, as it renders the file unusable. You have to convert the file into the file format you want it to be. This is the only way you'll be able to play the file after the extension is changed. Luckily, you can do this for free using online media converter websites and your Internet connection.
Choose an online media converting website that you wish to use (see References).
Upload your video file to the online media converter of your choice. Depending on the file size and your Internet speed, this could take a few minutes, so please be patient.
Select the file format you want to convert the file to under the "Output File Type" settings.
Do anything else required by the website. Certain online media converters (like Zamzar.com, for example) will require you to input a valid email address. This is so they can email you the file once it has been converted.
Press the "Start" button. This will begin the process of converting the file from one format to another. Once the process is done, the video file's extension will have been changed, but most importantly it will still work. You can now download the completed file (either from the web page or through an email) back to your hard drive.