Remove the mortar around the damaged brick with a mason's bolster and hammer. Chip away the old mortar by pounding the point of the bolster into the mortar. Stop occasionally to sweep out the old mortar with your fingers or a brush. As the space between the mortar and the brick grows larger, switch to a larger bolster. Be careful not to damage any brick you don't intend to remove from the stairs. Brick that becomes cracked or damaged will absorb moisture more quickly, causing the brick to become more damaged with time.
Remove the damaged bricks after the mortar has been removed.
Mix mortar with a mortar mixer according to the manufacturer's instructions.
Lather the new bricks with wet mortar, using a trowel. Insert the new bricks into the empty cavities where the damaged bricks used to be. Press the brick down firmly so the mortar oozes out over the sides.
Scrape away the excess mortar using a trowel.
Wait one week for the mortar to cure, then wash away any excess mortar with a stiff brush and water.