Determine the current format of your music or video and the format to which you want it converted (for example, MP3 to WAV for music, or MPEG to AVI for video). If you don't know the format of your file, you can usually find it by right-clicking the file and selecting "Properties."
Go to a reputable Internet download site (such as Cnet or Tucows---see "Resources") to locate a program that will convert your file from the current format to the desired one. You can type "codec" into the site's search box or the specific file types (example: "MP3 to WAV codec"). Some softwares are free to download; others might require payment.
Select a software and download it to your computer. Open the file and install the software.
Run the software and follow the instructions to select your audio or video file and convert it. Each codec software is different, so give yourself time to learn how it works.
Save the converted file to your computer. Preferably, you should save it as a copy, keeping the original file in case you need it, so you don't have to convert the file back later.