If the dent is in the plastic there is no way to push it back out. You will have to repair it with some form of plastic repair, I just use Dominion plastic filler its really flexible and sticks better than conventional filler.
But to undo the rear bar you need to:
~Remove the towbar if you have one
~Remove the 6 phillips screws in each wheel arch (3 each) holding the mudflaps and at the top.
~Remove the 4 (2 a side) 13mm bolts in the boot floor in front of the tail lights.
~Then slide the bar back off the clips.