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First, Why are you changing the bumper? Was it hit?
Loosen ALL bolts for the brackets and bumper. Mask all areas and use a floor jack w block of wood to get it where it needs to be, then tighten all bolts.
Post pics
My rear bumper was pushed in a bit on one side, so I parked it about 10 feet from a stout tree and hooked a block and tackle (come-along) to the tree and pulled it out that way.