Remove the fireplace screen, logs and anything else in the firebox that can be detached.
Take out the damaged firebrick panel or panels. If the rear panel is the one being replaced, it may be necessary to remove the side panels. Remove the screw that holds the retainer bracket and panels in place then carefully remove the side panels before pulling out the rear panel.
Measure the size of each panel and cut replacement panels to fit if necessary. A circular saw with a masonry blade should be suitable for this.
Reinsert the firebrick panels, beginning with the back panel. The back panel generally slides into a flange or bracket tabs. After replacing the back panel, do the same for the side panels by replacing the retainer bracket or clip and tightening the screw to secure the panels in place. If one of the panel edges is curved, the curved edge of the panel should be toward the front of the firebox.
Replace the log stand, logs, fireplace screen and anything else that was removed earlier.
Mortared Panel Replacement
Remove the fireplace log, screen, log stand and any other items in the firebox.
Remove the damaged panel or panels. It may be necessary to use a stone mason's chisel and hammer to completely remove the panels and mortar.
Prepare the area for the refractory mortar and panels. The underlying surface should be free of soot and any other debris. Use a moist sponge to dampen the walls and panels where they will be in contact with mortar.
Apply refractory mortar to the back wall of the firebox where the panel will be in contact with the rear of the box. In some cases, mortar should only be placed at the top and bottom of the wall to leave an open space in the center behind the panel.
Press the panel into place. Use a placeholder or brace to keep the rear panel in place or otherwise prop up the panel while the mortar cures. A wood board braced against the hearth floor or a lip at the front of the fireplace is suitable for this; a small but sturdy piece of furniture could also be used.
Apply mortar along the entire bottom edge of the front of the panel then apply a thin layer of mortar to the entire rear surface of the replacement side panel.
Place the side panel into the firebox. The side panel may be taller than the front opening of the fireplace so insert the panel into the opening at an angle and press it against the wall until it is flat. Consider using a rubber mallet to gently tap the panel into place. Do not tap the rear panel. Use a placeholder to keep the panel in place while the mortar cures.
Replace the second side panel in the same manner as the first if the other panel was also replaced.
Apply refractory mortar in the seams at the bottom and edges of the side panels.
Replace the log stand, logs, fireplace screen and any other items that were removed earlier once the refractory mortar has cured.