There are a couple of things to consider when replacing the RAM on a motherboard:
How much RAM will the motherboard hold (maximum capacity in GB)? how many sticks? what bus speec are they? what type of RAM are they?
Kingston Memory have a good tool for helping you select the right RAM for your make and model of PC. You can also google the make of your motherboard to see what the maximum RAM configuration is. Also, remember that windows 32bit editions can only handle upto 4GB RAM, so if you want to install more - you will need to also reformat your PC with a 64bit OS
When changing the RAM, make sure that you wear an ESD wrist strap - this will make sure there is no static (however small) built up on your hands which can damage the RAM of your PC.
To swap the RAM, unplug the PC from the mains and then hold the power button in (which will drain any leftover power in the computer capacitors). Remove the case and locate the RAM. It is usually stored at a 90degree angle to the main board and is slotted and clipped in place. Remove the old RAM and install the new by gently pushing it into place.
Replace the casing and power on your PC - that should be all that is required.
Some manufacturers require you run memory detection in your BIOS, but this is highly unlikely.
If you are still unsure about changing RAM, thee are loads of good youtube videos to show you how!