That becomes rather complicated. Would be best if you know someone who can help you with it.
You would first have to figure out if your motherboard supports more RAM(probably does, have to be pretty old to only support 4GB), if it supports Dual or Tripple RAM(probably Dual, tripple RAM wasnt really a thing for very long). Is it DDR2, DDR3 RAM(probably DDR3 unless it is quite old). Then you need to figure out which RAM frequency your CPU works best with. This RAM frequency then correlates to a number on the RAM like DDR3-1600 = PC3-12800.
You then buy (kit of 2x the same RAM block is recommended to take advantage of Dual RAM speeds) the RAM and replace your old ones with it.
Probably the easiest way to get a hint of which category to look in, would be to get the numbers of your old RAM and google it. As long as someone intelligent installed them, then it should match the stats of everything else.