As you may have figured out, the outside of the case only gives you access to replace one memory module. You’ll have to open it up to get to the other! Fortunately this is super easy to do. Just follow the instructions on this website. If you found the instructions useful, please consider making a donation.
If the site is unavailable, then you can download this pdf here which contains a printed PDF of the info you would have found on the site.
NOTE: The only thing I have to add is once you reboot the computer it will prompt you to make sure the RAM change was expected. Press F1 to continue at that prompt.