Generally speaking, there are slight indents either in the colored lens or in the base for a flat blade screwdriver to insert and twist to pop it off. You need to study it to find it.
You can see indents on this example.
Those gaskets are often inadequate. On my first motorhome, I removed the gaskets and used caulk to fill the pass-thru holes for the wires. You can also use a clear caulk around the outside of the base, but that can get ugly as the caulk ages.