Gigabyte B365 HD3 supports up to 64 GB of RAM in its 4 slots, that's up to 16 GB per slot. Try to get memory from the compatibility list - modules that aren't listed may work, but it's not guaranteed.
You can mix new memory modules with those you already have, just install new ones in free slots. In this case it would be best to get 2666 MHz modules, because all installed modules have to work with the same speed and in case of mismatch faster modules will underclock to match the slowest one. So buying slower modules will slow down the ones you have currently installed and faster ones will work at 2666 MHz anyway.
You could buy a 2x16 GB kit and replace current 2x8 GB, but you won't gain anything except for 2 free slots for potential expansion in the future. I'd just add another 2x8 GB, 32 GB in total is plenty of RAM. Going for 48 GB is also an option if you'll actually benefit from that much memory (some heavy workloads would, but for gaming it won't change a thing).
Installing RAM is easy. You have to unscrew the side panel, place RAM module in its slot and press the tab to secure it - done. I'm sure you'll find a tutorial on YouTube.