This videos show how you can open the back cover of HP 20 2115il All-in-One PC and then upgrade or replace your RAM (Memory) yourself in easy steps with just a screw driver.
You can see how many RAM Slots are there in HP All in One desktop.
HP 20 2115il All-in-One comes with 1.35v DDR3L 1600MHz SODIMM RAM.
If you need suggestion on RAM before you upgrade, comment and I will try to suggest best.