Hi,
According to this [https://www.crucial.com/compatible-upgrade-for/dell/inspiron-one-2305|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 [https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_inspiron_desktop/inspiron-one-2305_service%20manual_en-us.pdf|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