Place a ladder near the damaged wooden fascia.
Remove the gutters covering the fascia, if applicable, using a screw gun or impact wrench.
Pry the old wooden fascia away from the roof, using a pry bar. Pry off any nails that remain in the roof as well.
Measure the length of the area, using a tape measure, then measure and mark the new lumber with the tape measure and a pencil to cut it to size.
Put on safety glasses, and cut the new lumber with a circular saw.
Apply a coat of wood sealer to the new wooden fascia, using a paintbrush, and let it dry.
Fasten the replacement fascia to the roof, using a hammer and nails, and then reinstall the gutters, if applicable.