Put on safety glasses and gloves. Chip away the mortar on all four sides of the damaged or loose brick, using a cold chisel and hammer.
Remove the loose or damaged brick and then even out the empty space by chipping away the mortar, using the cold chisel and hammer.
Mix three parts sand with one part mortar in a bucket. Moisten the mixture with a hose and mix it together, using a trowel. Place the bucket near the area you are working on.
Squeeze concrete bonding agent into the empty space where the brick was removed, and then spread the mortar mix on the bottom and sides of the empty space. Set the old loose or new replacement brick into the space.
Add more mortar to the sides with a trowel, scraping away any excess with the trowel's blade as necessary. Tap the brick down with the trowel's handle to make it flush with the other bricks. Let the mortar dry for at least 12 hours before you walk on the brick step.