This story show how to replace damaged fascia.
Remove the guttering and trim, taking care it doesn't bend. Keep metal guttering away from power lines. Pry the damaged fascia from the rafter ends with a pry bar and pull out nails that stay in the rafters. Fasten a new length of rafter alongside the old one if the old rafter shows signs of rot.
Measure the opening spanned by the old fascia and cut new fascia to that length less 1/8 inch. Do not measure the old fascia -- its length may not be accurate, due to weathering. Prime the front, back, and ends of the new board. Nail or screw the replacement to the rafter ends.