The firebricks aren't glued or otherwise attached in the stove. They just set there. You can pull them out by hand.
Aside from refractory cement, I'm unaware of a method for repairing broken firebrick. A broken brick won't adversely affect stove operation if it's in position with no gaps.
If you'd prefer to replace it, firebricks are available from US Stove. They come in packs of six. We got a pack at Amazon last year to replace ones I broke by overstuffing the stove with too-big rounds. I won't be doing that again.