What exactly do you want to do?
Meaning, what is the final result you want out of the battery?
What voltage do you need it to be?
What charger do you have now? (if you lower the battery voltage, you'll need a new charger).
If you take cells out of the battery, or bypass them, you reduce not only it's voltage, but the total range of the system by reducing it's Wh capacity.
If the bike is a 72v system/controller, it probably won't operate at the lower voltage, so you'd need to replace it's controller, display, etc with ones meant for the lower voltage.
Note that anything involving working inside the battery, reconfiguring it, etc., has the risk of shorting something and starting a fire you probably can't put out on your own (without specific preparations for that event), or of electrocuting yourself, especially if you haven't any experience with this kind of thing.
If you just bypass cells in the battery as BatterGOLD suggests, by moving the main B+ wire, without replacing the BMS with one that only monitors the lower number of cells, and without getting a new lower-voltage charger, then charging it is going to take a VERY long time, as it will have to continuously bleed tiny amounts of charge off the cells still in the pack but not being used, so charging will start, run for a few seconds or minutes, stop, drain for minutes or hours, then restart for a few seconds or minutes, and repeat this cycle for hours or days (or longer) until the in-use cells finally finish charging.
Using a lower voltage charger on only the still-used cells will fix this after the initial time the BMS has to drain any of the higher cells of a bit of charge, and be normal on any subsequent charge.
The advantages of just bypassing the cells is that they are still in the battery if you need to upgrade later, and you are not disassembling the battery physically, reconfiguring it, etc., so it's safer.
Disadvantage is the cells are still there taking up room and weight.
The BMS doesn't protect the bike or controller, etc., it is there to protect the cells. Which BMS you get will depend on what the cells are capable of vs what the controller itself needs.
What BMS is in there now, and what current (amps, A) are the cells rated for?
What controller does the bike have now, and what current (amps, A) is it rated for?
