How do I remove these ceiling diffuser flaps?
You need to access that section from the attic and pull the screws then the assembly will drop down. You have an older damper from the days when they were not made to be removed so that ~1’ section of pipe with the damper needs to be removed. I am guessing at the 1’ but you can see the screws that connect the 90 to the damper. These are the screws that need to be removed. I think I see 3 total and there are usually 3-4.