You are telling us very little, what exactly you are trying to change, what kind of music, what purpose.
Depends on what you are trying to achieve. Two options I think:
1) temporary changing the BPM in order to (beat) match another track
2) permanent changing of the BPM because the software analysed it wrong
1) This you do with your pitch fader or pitch buttons. As soon as you load the track again next time that information is gone. If this is what you are trying to do, I really don’t understand why. Every time you have to make adjustments to BPM of course, depending on track mixing out of and track mixing in to.
2) This of course is easier and makes more sense. When you analyse a track RB will set BPM and corresponding beat grid. If the BPM is incorrect, then the beat grid will be incorrect as well. You can go into beat grid mode and change the beat grid manually. This will change the BPM too. The other way around works also, if you change the BPM (you can use the tap button to get it close to right) the beat grid will change. This information will be saved by RB and used next time. Careful though, if the beat grid is correct and you want to give a track another permanent BPM and change the BPM value from what the track originally is to something you want, it will move the beat grid, so that will no longer match the tracks beat, making it impossible for sync to work properly.
Finally there is one very common reason to want to change BPM and that is the 1/2 or 2x mistake software makes. Sometimes the software can’t figure out the correct BPM and will report the speed as half or twice what it really is. So your 150 BPM track might be reported as 75. Or a slow track at 70 might be reported (and beat gridded) as 140. You can set the range that RB uses and if you have this happen a lot because of the genres you play, you might want to change the default range to something that better fits your needs. Individual tracks can be corrected in beat grid mode by clicking the 1/2 or 2x button. It will automatically reset the beat grid for you to match the new tempo.
Hope that helps.
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