Choose a warm day. The shingles are more pliable and easier to handle.
Locate the leak. Use a pair of binoculars to spot loose shingles on the outside. You can also look in the attic for signs of water leakage.
Repair minor cracks or holes with roofing cement. If the damage is more extensive, proceed to the next step.
Start with the row of shingles above the damaged one. Holding the shingle tab out of the way, gently work the nails loose with a flat shovel. Pull the nails out. The shingle that needs replacement is now exposed. Remove it.
Cut the replacement shingle to fit. Depending on how much you remove in the previous step, you may not have to make any cuts. Or, you may need to cut two shingles to space the shingle tabs correctly.
Slip the new shingle in place. Drive nails into the old holes if possible. If not, seal the old holes with roofing cement before nailing the new shingle in place.
Coat all new nail heads with roofing cement. Press the shingle firmly into the roof to make sure the shingle's adhesive is in contact with the roof. The sun's warmth finishes the job of sealing the replacement shingle to the roof.