Sounds like the OP is not talking about the actual rear bumper but the "aluminum" colored bumper cover/guard.
If that's the case, the best way to remove it would be to either park it with the sun hitting the rear for a while or ideally use a hair dryer or heat gun to gently heat the cover (you are trying to heat up the adhesive tape that holds it down) making it easier to remove.
Then get some floss (NOT metal fishing line!) and slide it from one edge to the other underneath the cover, to cut through the adhesive tape. At the same time gently pulling the cover up as you move along.
Same idea as removing badges or emblems from cars.
If that's the case, the best way to remove it would be to either park it with the sun hitting the rear for a while or ideally use a hair dryer or heat gun to gently heat the cover (you are trying to heat up the adhesive tape that holds it down) making it easier to remove.
Then get some floss (NOT metal fishing line!) and slide it from one edge to the other underneath the cover, to cut through the adhesive tape. At the same time gently pulling the cover up as you move along.
Same idea as removing badges or emblems from cars.