Insert a small pry bar between the fascia board and the backer board. Gently tap the pry bar with a hammer and begin prying the fascia board away from the backer board. If you will reuse the fascia board, pry it out just enough to move the pry bar down the length of the board.
Continue prying the fascia board in small increments until you are able to see all the nail shanks between the fascia and backer boards. Pry where the nail shanks are located until the fascia board is off the backer board.
Insert the crowbar between the backer board and the framing. Pry the backer board away from the framing as far as possible. Move the crowbar down the length of the backer board, prying it away from the framing. The backer board usually has 16d nails securing it to the framing. Continue prying with the pry bar until the backer board is off the framing.