How do I remove this?

My brother accidentally installed these two bricks, how do I remove them?

Does anybody know what Lego set this is from?

I’ve long since lost the instructions, but not my Lego collection. I probably built it 25+ years ago (I’m 37) and it’s been broken and repaired so many times it’s drifting from its original design. I’d like to fix that.

I have this green baseplate and I am afraid of cracking on minifigures what do i do?