Both Realband and the free program Audacity offer "change tempo without changing pitch" against a WAV track using a percentage adjustment. That's the good thing.

The bad thing is that there will be a rounding error. If the track is long enough, you may notice loss of synchronization. For example, to slow the track from 88.5 BPM to 88 BPM you'd need to tweak by the percentage 1.005681818181818. Somehow I don't think the result in either program will be quite that precise.

In either program the process involves importing the WAV file, selecting the track, applying the "time stretch" effect, and specifying the desired percentage change. Using "time stretch" in either program will be difficult for you. If you think the result will be close enough with a rounding error, and you can commandeer someone to help you, come on back and we'll try to be very careful in laying out the instructions.

Good luck.

-Mel