Make a solution of distilled water (not tap water!) with baking soda 1 tbs per a ml. Let the solution sit a bit so any remaining solids settle to the bottom.
Carefully open the upper case following this IFIXIT guide MacBook Core 2 Duo Upper Case Replacement.
Using a cotton swab (Q-Tip) and baking soda solution carefully wipe down the surfaces that you see any staining. After wiping dry the area with a clean dry cotton swab. Go over the areas again this time with straight distilled water to clean off any remaining baking soda and again wipe dry. The last step you want to go over the areas again using a cotton swab with good grade of Isopropyl alcohol (85% or better) this will act as a drier cleaning up any water that has crept under anything.
Follow this vid on popping off the key caps How To: Replace or Clean your MacBook, MacBook Pro, or Mac. Using the same process as above carefully wipe down the areas that got wet. The last step is to take the keyboard apart. Sorry to say there's no good write-up on this part but it should be straight forward taking the screws out and carefully separate the plates.
If you don't want to give this a go you could just replace the top case.