The ram in my XPS 13 9360 is not upgradeable. The ram in the XPS 13 line (9350 / 9360 anyway) is apparently soldered to the motherboard. As already mentioned, something to do with how thin Dell made the laptop.
Interestingly enough, my Mobile Precision m5520 (XPS 15 equivalent) does have upgradeable RAM. It has two SO-DIMM slots easily accessible once the rear panel is removed. (Just use quality Torx-head screwdriver to remove the screws so you do not strip the heads.)