You don't need to remove the shock tubes to replace the bumper as you can just push them back out, of course I have no idea how hard the back end was hit, so you may need to remove and replace them.

To do so, there are 6 bolts total, 2 on each end of each shock tube, and 1 on each side that come down from the inside of the car. You get to them by lifting up the rear floor compartment and you will see a large hole on each side of the compartment. If you look down into the hole you will see the top of each bolt head. Both will need to be removed so that you can remove the shock tube and the replace it.

My advice on replacement is to put the shock tubes in first without the bumper and then get the single bolts started before you secure the two bolts at the rear of the shock tube.

Once the shock tubes are secured, then you can easily put the new bumper on.

Hope this helps, and good luck.

Matt