That door comes off really easy, if you open the door and look at the inside of the hinge there is a silver thing that looks like a screw. With your finger nail pull that toward the bottom of the camera then gently rock the door and it should tilt away from the camera so that the top of the door is free. Then just lift the door up to free the bottom hinge pin.
Assembly is the reverse of assembly.
I would sell you my N90s with a nice not sticky door, but the door is all scratched up.
FYI, I paid $65 for my N90s with a scratched but not sticky door.