According to this link 8GB (2 x 4GB DDR3L- 1600 UDIMM) is the maximum amount that the computer supports.
Most Win 10 PCs work reasonably well with 8GB of memory but not if you’re into gaming.
Here’s the service manual for the computer. Click on the memory module(s) link at the start of the manual, to view the necessary pre-requisite steps and then the procedure to replace the memory modules.
Since you have 6GB already I assume that it is 1 x 2GB + 1 x 4GB so if you only replace the 2GB with a 4GB make sure that both the 4GB modules will have the same specifications. You should be able to find them on the module itself