how do I go about accessing the existing backup bulb?
For the 2018, instructions start on page 11-42 of the owner's manual. There are also some videos on YouTube somewhere. Basically take those two screws out, slide the housing towards the back of the car and it will slide off with a click. Don't slide too far because wires are attached. Then using your fingers you unscrew the socket of the light of interest from the housing to access the bulb. When you put the bulb in, try to make it turn on before (press a pedal, click the stalk, move the shifter, etc depending on which light you are working on) you put the housing back on, because the bulb might be in backwards and you'll have to remove it from the socket and turn it around.
The first few times I took the rear light housings off I just stood beside the car and pushed the housing backwards, but this isn't the best way because you will be pushing the housing towards the car, and as it unclicks, the pointy end towards the front will be pushed against the car, scratching the paint. Instead, move the housing straight back without pressing it in any sideways direction. A better way to do it is to take a metal coat hanger and make a small hook in the end. Undo the two screws, and hook the coat hanger around the housing flange where you removed the screws from, and gently pull the housing backwards. You can use a Philips screwdriver or a small socket to remove/reinstall the screws.
You can download the owner's (and other) manuals from somewhere on Subaru's websites. It's much easier to flip through and search the PDF than it is to use the paper manual.
