In order to replace damaged roof shingles you need to remove the damaged shingle install the new one and seal the edges.
How to replace damaged roofing shingles.
How to replace a damaged shingle.
In the photo below the target shingle perhaps it needs replacement due to damage from a falling object or roof pests is located.
If the existing shingles are brittle you may not be able to pry out the second row of nails without cracking a shingle.
How to replace a damaged roof shingle without changing entire section of roof.
After yanking all eight nails pull out the damaged shingle.
Asphalt roof shingles can be easily damaged by high winds and falling tree limbs.
Removing 3 tab shingles using a flat bar to do a repair.
Removing 3 tab shingles using a flat bar to do a repair.
Use the claw of a hammer to loosen the adhesive under the tabs 2 rows above the damaged shingles and then loosen any exposed nails with the claw.
We had a wind storm recently blow off one shingle and damage 3 shingles on our garage.
In that case tear out the damaged shingle and cut v notches in the replacement to fit around the four nails.