If you already know how to set the wanted level (i dont know how haha) but if you know the function, lets say its just wantedlevel for now
You would probably normally do wantedlevel = 5 for example, that would then set the wanted level to 5 stars.
Instead try to do wantedlevel += 1 . That should add 1 to the existing wanted level. Then to remove one star do:
wantedlevel -= 1 that would then remove one from the current wanted level
the += and -= operators will append and deduct respectivly. Instead of simply setting the value to whatever you say after the operator.
Hope this helps. Im not 100% sure you actually can append and deduct on the wanted level but hopefully you can