Just twist the covers off. If that doesn't work, try getting a small screwdriver under the edge to pry it off. They are a little tight on there.
The speaker itself should be held with three square head screws. The hole in the ceiling will be pretty big, so plan on a big speaker or some type of patch. The speaker size is just about what you see looking up, maybe minus 1/4" for the plastic cover in terms of diameter.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR