Apple Service, and an original apple RAM for what matters, are quite expensive.

It is really useful if you can do the change or upgrade yourself - and it is easy!!!

Just be careful if it is the first time you do so, be gentle...

Check the video if you want to "see" the process. happening.

Step 1: Dismantling, Replacing, Putting It Together

Follow the pictures / video steps.

It should take between 5 and 15min, depending on your expertise.

Essentially. A macbook is screwed together. Even "unibody"s are screwed together.

Find a screwdriver that fits those small screws and be gentle with them - you don't want to break them. (Tip: keep all the screws in the same place or container, such as an emply glass of water.. you'll need them!)
Notice that there are 3 longer ones - remember where they were screwed.
Remove the cover, and locate the RAM slots.
Gently remove the original RAM from the slot - you will need to "open" the slots a little, there are plastic bars that can be gently open wide, and the RAM should just "pop out". Simply help them, and then remove them.
Place the new RAM in those slots.
Put the cover back, and don't forget to screw each screw at each place!

You're the prowd owner of an upgraded MacBook now!