Remove the boiler cap using the group handle as per the instructions.
When tightening the boiler cap tighten by hand rather than using the group handle if you feel significant resistance it could indicate that the seal is sticking to the boiler mouth and might tear so try lubricating the seal by rubbing a little cooking oil onto it.
The boiler cap should always be tightened until it hits the stop to get the optimal seal.
New boiler cap o-rings are available here.