Light bulbs in head: Remove the two plexiglass pieces (if your game has them) at the top of the playfield, followed by the chrome plastic helmet (you don't need to disconnect from the wiring harness -- there should be enough slack in the cables to just move the helmet to the side). At this point, you can remove two screws that attach whatever face is showing to the face frame. Now you can access the three bulbs. This is going from memory, but I'm pretty sure this is correct.

Pop bumpers: you do have to remove the Pinbot playfield. There's no way around it. You will probably have to remove the two plexiglass pieces and chrome helmet first, as described above. I seem to remember there are only three screws holding the playfield down (two large ones and one smaller one on the bottom right).