My dog has broken/chewed the door flap, how do I replace it?

1. Remove the outer frame with the flap attached
2. Remove the screws at the top of the frame securing the flap
3. Remove the flap clamp bar/strip and remove old flap
4. Position new flap in frame, replace strip/bar and secure with screws
5. Screw the pet door frame back in place

1. Unscrew/remove the frame with the flap attached from the area it has been installed into.
2. Sit one flap hinge in place and then wedge a screwdriver between the long edge of the flap and the inner edge of the frame to expand the frame enough for the other hinge to slot back in. 

Please note when leveraging the flap in and out of the hinge hole it does require some force but as the plastic flap is slightly flexible, it should be able to take the pressure.