If shingles are cracked, missing or rotten, replacing them may be another simple fix (provided you have the replacement shingles). To remove a damaged shingle, lift the edges of the surrounding shingles and carefully remove nails with a pry bar. Once the nails are removed, the shingle should slide out. Scrape away any of the residue cement from the roof and level or remove protruding nails. Before you attempt to replace a new shingle, round the back corners with a utility knife. This will make it easier for you to slide the shingle under the one above and align it with those on either side. Once you have it in position, lift the corners of the overlapping shingles and fasten the top of the new replacement with 6d galvanized roofing nails. Be sure to secure with nails in each corner. Last, cover the nail heads with roof cement and smooth down the overlapping shingle edges.