How do i replace the rear turn signal bulb

There are black plastic wing nuts behind where the light is, there is usually 3 or 4 of them, remove them and the whole light capsule can be removed and give you access to the bulbs

That sealed look has nothing to do with the tail lamp. It's just a covering for the welded seams of the metal end panel to the quarter panel. Almost fooled me too when I replaced mine. Thought someone had performed a backyard body repair with JB Weld. After you remove those wing nuts, there are three studs with 10mm nuts - remove those three nuts and the lamp will come away from the quarter panel, revealing the bulb sockets. The sockets have a little tab you must squeeze to be able to to twist it out from the housing. Squeeze and turn counter-clockwise. Like some medicine bottles.

Mine keeps blowing in a 2000 Buick Lesabre how do i keep it from blowing? No moisture gets into the assembly itself though should i use bulb grease or leave it be?

Stop slamming the trunk.