To replace a fireplace surround, first remove the molding on the inside edge of the existing fireplace mantle by scoring the strip of wood with a utility knife, wedge a putty knife in between the molding and the fireplace, and carefully pry the molding off. Use a pry bar to remove any remaining fireplace surround. Install the new fireplace surround with construction adhesive, and then install new molding.
When removing the surround, wedge the pry bar between the surround board and the wall. Apply pressure to the top of the surround leg, move the bar toward the wall to slowly break the existing adhesive bond, and lift the surround from the fireplace. Use this process to remove both legs of the surround and the top piece before using the putty knife to scrape off any residual dried adhesive from the fireplace front.
Apply two lines of construction adhesive to the surround. Press the adhesive to the front of the fireplace, and allow a couple of minutes for it to bond to the wall. The top pieces of surround are typically large than the legs, so apply a third layer of caulk between the lines of construction adhesive if it fails to stick. Use a hammer and nails to re-install the fireplace molding.
If necessary, install fireplace doors. Finish the surround with paint, veneer or stain as desired.