Lets review what your system has for RAM slots: 2 x SO-DIMM which allow it to have two PC4-21300 DDR4 SO-DIMMs. When you ordered your system it came with two 4 GB SO-DIMM’s giving you 8 GB total RAM.
Your system can support upto 64 GB of RAM using two 32 GB SO-DIMM’s. You could take out one 4 GB DIMM’s out and put in a 32 GB DIMM that would get you 36 GB!
Here’s the iFixit guide you’ll need to follow [guide|115309]