That model has 2 RAM slots. It supports a total of 16GB.

The note that the slot is non-accessible is just a standard note that HP puts on their laptops when you need to remove the back cover of the chassis to add in more RAM.

They prefer you to take the laptop to a service center to do the upgrades rather than do it yourself. I tend to ignore it though. If you're comfortable doing it yourself, the RAM slot is right in the middle upon removal of the back cover.

One thing you have to becareful of HP laptops when you increase the RAM. I am not sure of your particular model but a lot of the newer HP Pavillion models have little plastic hook like things on the bottom base.

When you open it, removing the screws is not enough. You have to use something to unhook it like a credit card or a small screw driver. When you break these hooks, HP will claim that you tempered with the laptop, causing a warranty void.