Search online for unroot galaxy s2 and follow the directions, this is on the galaxys2root site. I rolled back mine to 2.3.4 without any issues. Everything seems to work including wifi, and battery life is back to normal. 2.3.6 sucks!
Forgot to mention that I have a stock phone, never rooted.
Another point: I didn't need to go to step 8 in the directions to return the phone to factory defaults. I stopped at step 7, so all of my apps and settings were preserved!