How to replace keyswitches?

I have a Model 01 keyboard with two defect keys, one on each side. I bought replacement keys, but I hit a snag: I can’t remove the baseplate that is keeping all in place.
How should that big black plate be removed?

I have to get it away, because the key I need to remove has solder on both the bottom AND the top of the board.

While I’m asking for helping, are there any more tips for replacing keyswitches?

I think it likely that they will reply to you here, but you may want to email