I am not certain how 4.2 RAV rear bumpers are held together, but they look similar to our 4.1s. So I'll attempt to answer your question; however, there may be a few differences.
Because it is the rear left (drivers side) bumper, it is much easier because you do not have to deal with the hinged door interfering with some bolts that you need to remove.
Anyhow, when you open your rear door; you will see 2 bolts that attach the rear of the bumper to your RAV. You will need to remove these. Also, look under the rear bumper and you will see 1 or 2 more bolts to remove. There may be a bolt that needs to be removed if you have a mudguard bracket in place.
Look in the inner rim of your wheel well, and you will find about 3 or 4 screws that need to be removed. This holds the front of the rear bumper in place.
Also, don't forget to unscrew the rear sidemarkers from the bumper; you will need to remove these anyways to replace onto your new bumper. Dettach light socket wire.
After you remove all of these screws/bolts and it appears that nothing else is holding the bumper to your RAV, all you need to do is yank it carefully from your RAVs body. The bumper is being held on to the side of your RAV with some large ball/socket clips (2); and should be able to be yanked carefully off. I say carefully, because you do not want to break the plastic clips.
That should be it, and installation is the reverse of removal. Good luck.