What you will need is a putty knife and screw gun to pull the old fixture off. Scrap the head of the screws coverin sealant off enough to get the "Roberts",(square headed) bit into the screws and back them out. Once all the screws are out then start at one corner and get it started up. using a strong putty knife you should be able to get it off and scrape off only what you have to. Clean and install new vent and reseal with new lap sealant. Good luck and let us know how it went.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7