Measure the width, height and depth of the old mantel and use these measurements for installing the new mantelpiece.
Pry off the old mantelpiece using a pry bar. Slide a putty knife between the mantel and the wall to protect the wall from the pry bar.
Take another measurement of the width and height of the fire pit opening and use this for ordering the new mantel.
Attach the new mantel pieces together using the screws and instructions provided with the mantelpiece.
Push the mantel into place and make sure it is plumb all the way around using a spirit level. Place shims (scrap pieces of wood) between the mantel and the wall to make it straight if there are uneven points. Cut the shims so that they are flush to the mantel.
Fill in the gaps around the mantel using caulk. Let the caulk dry according to the instructions on the label. Paint the caulk to match the mantel.