I doubt over filling would make it boil. To drain a bit of fluid you can either Google the thep of car and find the bleed nipple and see if anything comes out with the engine cool and running. If not then you would need to loosen off a lower pipe and let a little drain out but not much.

I would then be looking for the source of the problem. These things can be a bit frustrating to find. Can you post up the make and model number including the engine size and type as it could be common.
If in doubt seek a pro