A fireplace is not a very clean home feature. All the built-up soot, hearth stains, and chipped bricks or stone can make it pretty unsightly. With a few days work, though, you can tear out that old hearth and mantel and replace them to make your fireplace look brand new. You should have an extra hand for this project, as a lot of the pieces you'll be dealing with are pretty heavy.
Out with the Old
The first step of this project is to tear out the mantel. Do so using prybars, and be extra careful if you intend to reuse it. Building codes require that all wood framing and lath are a minimum of two inches away from the opening of the fireplace (firebox opening). Make sure this is true of your fireplace before moving on.
Next, remove the old hearth. Remember to wear eye protection and a breath mask during this part. Protect the surrounding floor by laying cardboard over it and taping it down. Fit a demolition hammer with a chisel bit and use it to chisel away the old hearth and mortar. Be careful, though, not to damage the concrete beneath it. Get rid of all that debris you just created by sweeping it up and vacuuming whatever is left.
TIP: Now might be a good opportunity for you to give your fireplace box a thorough cleaning.
Installing the New Hearth
First, you'll need to prepare the hearth base (after buying it, of course). Get a piece of 2x4 that's about a foot longer than the base area. Cut notches, six inches long and as deep as the new hearth (or as deep as you'd like the new hearth to be set), into the ends of the 2x4. Pour concrete into the hearth base area and pull the 2x4 (called a screed), with the notched side facing down, across the concrete to smooth it out. This will ensure that the concrete is set at the depth necessary to make the hearth level with the surrounding floor. Use a trowel to further smooth the concrete. Let the concrete set overnight.
Once the concrete is dry, you can test fit the hearth slab, using suction cup handles to pick it up. If it seems level and flush with the rest of the flooring, remove it. Then use a toothed trowel to spread thinset mortar over the hearth base. Then, using the suction cup handles again, carefully set the hearth slab into place. Press down on the slab to adhere it to the mortar. The mortar won't set instantly, so if the hearth slab rocks or is set too deep, you can remove it and add some more mortar. If the hearth slab isn't set deep enough, use a rubber mallet to tap it down. The gap between the hearth slab and the firebox floor should be filled with thinset mortar and smoothed out.
Installing the New Surround Pieces
As with the hearth, you'll need to buy new surround pieces for your fireplace. Spread thinset mortar onto the backs of the sidepieces. Set each piece on top of the hearth and against the brick of the fireplace. The inside edges of the sidepieces should overlap the sides of the firebox just slightly. Put a level across the top of the sidepieces and shift them until they're level. Then, you can spread mortar onto the top piece and push it into place across the top of the sidepieces. Check for levelness once again. Make sure the joints between the sidepieces and top piece are flush. Let the mortar set overnight.
Replacing the Mantel
The last step is to replace the old mantel or install a new one. Place the mantel on the hearth and butt it up against the new surround pieces. Since the surround pieces stick out slightly from the wall, there will be a gap between the wall and the mantle. Measure that gap and then cut to cut wood filler strips to that thickness. Pull the mantle out again, and glue and nail the filler strips to the back of the mantel, flush with the edges. If you are working around floor molding or a wall that's bowed, you'll need to scribe the back of the mantle using a compass and then cut along the lines with a jigsaw.
Finally you can center the mantle over the fireplace, push it firmly against the wall, and toenail it into place using 8d-finishing nails. Then, with a new coat of paint on the mantle, you're fireplace will be looking like new again.