Changing the key of a piece of music is called transposing the music. Transposing music is not the most difficult element of music theory, but it does require a mastery of the various key signatures and modes to be done correctly. Beginning musicians needs a simple tool that could transpose backing track to put it in the right key for vocalists.
AudioRetoucher allows you to change the key of any MP3 file without changing the tempo.
If your music is already stored as MP3 or WAV file, AudioRetoucher will transpose it for you. However, if you only have the music on paper, it may be easier to transpose it yourself than to enter it into a music program to have it transposed.