I’m going to provide the CliffsNotes version answer to this question - until I get a chance to sit-down and go over this w/ more detail.
How to remove a fire brick
How to remove a damaged fire brick
How to remove a cracked fire brick
On the fire brick you want to remove, drill 3 holes (red dots).
Then hit the fire brick w/ a hammer.
It should clean break into 2 or 3 pieces for easy removal.
If it isn’t easily removeable. Drill 2 more relief holes (blue dots).
Whack it with a hammer. You should have several small pieces now.
Remove the broken fire brick… adjust the sand (that was under the fire brick), then place a NEW fire brick into the void and tap flush into place w/ a rubber mallet.
NOTE: Once that broken fire brick is removed, you will notice how easy it is to pick-up / remove all the other fire brick on the hearth surface. Unless you absolutely need to adjust other hearth fire brick, try to refrain from removing any other hearth fire brick. One that Genie is out of the bottle, it’s VERY hard to get all your fire brick back into place.