Top part of the check valve on the bottom. there is a fine screen that acts as a filter. Remove that by removing the three Phillips screws with a long #2 Phillips screwdriver. Clean the screen under running water with a brush. Reassemble after proper cleaning.
Next step is to remove the top cover to get to the water supply to the K-cup. Open the Keurig. remove two Phillips screws that hold it in place.
Lift the top cover up and forward to remove it.