Lenovo Ideapad G500S is an amazing laptop and just like most of the Ideapad laptops, it is very easy to upgrade the hard disk (to bigger HDD or SSD) and RAM. To upgrade the RAM, you will need to buy yourself DDR3L chips.
To upgrade either RAM or hard disk on the Lenovo Ideapad G500S, first unscrew the three screws that hold the back panel. Make sure you have removed battery just to be on the safe side. Slide the panel in north direction to take it off. On the top left side you will find one or two RAM chips depending on your laptop configuration.
Carefully release the latches on the side of the module holder and lift to around 30 degree angle from the bottom surface of the laptop. Now slide it out and insert new chips.
You can get the DDR3L chips for cheap from here. I recommend getting two 4GB or single 8GB RAM chip to boost performance.
For the hard disk, unscrew the four screws that hold the HDD caddy and slide out the hard disk. Insert the new one and place the caddy over it.