It's not really clear from your question whether you only want to change the playback speed (like the pitch fader on a turntable does) or whether you want to change the pitch without affecting the playback speed. If the latter, you need to use a time stretching/pitch shifting algorithm. See: http://en.wikipedia.org/wiki/Audio_time-scale/pitch_modification
In any case, you'd first have to decode the MP3 into PCM audio. This thread might give you some pointers:
How to get audio data from a MP3?
As for doing pitch shifting in Java, there's some more info here:
Slowing down the playback of an audio file without changing its pitch?
