Replacing a damaged bricks is important as they are exposed to the elements and all kinds of weather. If a brick is broken or cracked, it should be replaced, because the opening will be allowing moisture to get inside your walls. Now, replacing a brick in a wall (or a chimney) sounds pretty intimidating, but it’s really not that difficult a job. You do need a couple of special (but not expensive) tools, which will cost less than hiring a mason to do the job. Here’s how you can go about replacing a damaged brick.
Finding a replacement brick to match the color of your wall may be your toughest job. If your brick is only cracked and not totally broken, an option may be to remove it intact, then turn it around and face the good side out.
Once the brick is loose, remove it, and if you're going to be reusing it, place it in a bucket of water. If you're using a new brick as the replacement, it needs to be soaked in water for at least an hour before installation as well.
Spray the bricks around the hole with water (to stop them from absorbing moisture from the new mortar and drying it too quickly), then spread a thick bed of mortar on the bottom of the hole.
Use the handle of the trowel to make the new mortar joint match the joints in the rest of the wall. Allow the mortar to dry slowly by spraying it with water a couple of times a day for three or four days.