Grab the top of the solid panel cover and pull it out of the door channel if in place. The cover is the panel you slide in to prevent your dog from going through the PetSafe doggie door.
Open the door and place masking tape over all four corners of the outer doggie door frame. Extend the tape past each corner and attach to the door. This helps keep the outer frame of the doggie door in place while you replace the doggie door flap.
Close the door and remove the nuts securing the inner door frame to the door with a socket wrench. Pull the inner door frame away from the door slowly. The doggie door flap hangs from the threads of the bolts still in the door.
Lift the doggie door flap off the bolt threads. Install the new doggie door flap, inserting the bolt threads through the holes in the top of the flap.
Reposition the inner frame back onto the door, inserting the bolt threads through the holes in the inner frame. Thread the nuts over the bolt threads by hand. Once the nuts are hand-tight, tighten them completely with the socket wrench. Remove the masking tape from the outer doggie door frame.