If you wish to remove an existing breaker it simply tilts toward you (you need not remove wire first).
If you wish to ADD a breaker you may need to modify the cover.
True story.. i was working at the Moose lodge thy ask me to remove a dual-ganged breaker that used to feed a garage that got vandalized by an arsonist (Burned to the ground) I removed the wires Grabbed it with a gloved hand (Shock protction just in cuss) and nothing moved..
Finally I walked back across the lot to the elecrician and told him it was rusted in place.. He ask some question and I held up my big channel locks in one hand and the dual breakr in the other and said "It was not, however a problem".