how do I get it open to change the bulb (sorry if this question is as stupid as it sounds)?

 

Which ones? There are two different groups on either right or left of your car. There's the ones on the outer panels and ones on the trunk.

 

Well, this question was even stupider than I could have imagined, because I forgot to say which tail lights they were (the fender ones), and then of course I figured it out shortly after posting: open the small round flap, insert a screwdriver and unscrew the piece holding the light assembly in, and then it is easily disassembled for replacement.