Hi Terry,
You can't edit and save mp3's on Tyros 5, but there's a way around it.
Simply load the song you desired to play, do the transposing and save it as a Registration. Just make sure you put the check mark on 'Song' tab, and anytime you recall that Registration, it will automatically load the song WITH the transposition applied.
Although I wouldn't suggest transposing more than 2 semitones up or down due to significant sound degradation, and probably it will be better idea to start looking for another mp3 which is nearer to the desired key.
Cheers.
Valentin