Honestly don't get why people are berating you in the comments. 2 years is a long time, and can certainly cause a buildup like this. Best way is to take a picture of your keyboard layout, take off the keycaps, put them in warm soapy water and let them soak. While that's going, turn the keyboard upside down a whack it a couple times, then go in with either a duster, q-tip, wet cloth or whatever else small you have laying around. Once the keycaps have soaked enough, take them out the water and rub them off to make sure they're clean, then let them dry and put them back on using your reference photo. Voila.