How do I replace my keys?

you will have to buy a new keyboard cleaning them is possible but very tricky most likely a very used keyboard

@alistairmoran @bnuyo the keyboard on this model is part of the upper case. You may find one that is separated, however, you will have to remove the old one form the top case. It is held in places by a lot of plastic "tabs" coming of the top case. Once you got it removed you will have to convert all those plastic tabs to holes that will take very small screws to hold the keyboard. It is most likely easier and less prone to failure, if you replace the top case if you wan tot change the keyboard.

This is not to say that you can't fix the keycaps on your computer. Take a look on here and see if that will help you out. Different model but the keycap replacement should be very similar It may help us if you post some good pictures of what you got going on there with your E and R key iFixitでの質問に画像を追加する方法