If it is user accessible ram you can just purchase a larger stick and replace it
but you will want to make sure what it has and what you can access
for instance, my ultrabook came with 4gb of ram
one slot I can get to
the other is some sort of hardwired ram or a slot that you cannot easily get to
So run a 2gb stick and an 8gb stick for a total of 10
Some laptops you can get to both slots
some that have 2gb will just have a single stick, in which case the upgrade to 4 is cheap, and the upgrade to 8 wouldn't be too much more than that