it's either going to be a clip or a screw that hold the cap on.
the clip is obvious, once you pop the clip off the cap flange it falls out of the way
as for the screw type, alot of caps that use the screw method have a retainer holding the screw onto the cap so it wont fall out, some are simply just a rod with a bent end to catch underneath on a ledge so when you turn the screw it just unlocks.
If your having trouble getting the cap off make sure the screw is actually all the way out, if it is the bent rod type make sure it is off of the ledge it catches, otherwise it may just be a tight fit and bumping it to one side may get it to come off.