Removing a circuit breaker

In my TT I have a WFCO converter with its associated breaker/fuse panel. I want to add another breaker for a new converter, but as I was looking, I could not see a way to remove an existing breaker. I had the cover removed. I could see the lip to hook the new breaker over and the contacts for the supply. But, I couldn't see how one would pull one.
I know it is probably something simple, but I didn't want to start prying and crack one of the existing breakers.
Any help would be appreciated. Thanks in advance.

Mike