The rubber part is usually not glued down so it can be lifted rather easily. However, there may be some silicone type grease between the rubber boot (that looks like a suction cup) and the CRT glass to seal out dust.
A metal clip with a spring keeping it spread out attaches inside the button.
While there are a variety of types of clips actually used, pushing the connector to one side and/or squeezing it in the appropriate direction (peel up one side of the rubber to inspect) while gently lifting up should free it. Probably :-).
The clip (when removed) and CRT button look sort of like this:
||======= HV Cable /\ Clip | | (Removed) _| |_ (No DAG coating in vicinity of HV connector) ____________.- -.___________ CRT ____________|______|___________ Glass Metal Button
Replacement is done in reverse order!
This isn't rocket science and excessive force should not be needed! :-)