It just turns until you find a spot where it will come out. Usually very stiff to turn and need something as leverage. If it breaks buy a new vent cover. Not that expensive. More importantly is why you want it out. What are you hoping to achieve? If you think about the problem logically, the vent can't move closer to the turning fan. The fan can come loose and move forward until it touches the vent,so the problem would be caused by the motor side not the vent. I would be taking the motor out and inspecting the fan and seeing if it can be moved back slightly on the shaft. It is only held on shaft with a grub screw.
Lincoln Appliance NZ, January 2018