To replace the keyboards one needs to completely replace the upper case as its part of the assembly. Review this IFIXIT guide [guide|5907|MacBook Pro 15" Unibody Early 2011 Upper Case Replacement]. In addition you will need to take the battery out as well (the guide fails to point that out).
While some people have managed to get the keyboard assembly alone to replace their damaged one its a lot of work as you have to deal with aprox 50 screws and/or rivets or both taking them out and you tend to break the snap rivets so you'll need to replace them with the correct screws.
Your other option here is first trying to clean the key mechanism under the key cap. Follow this YouTube vid [http://www.youtube.com/watch?v=jSpPlMdGvVI|Taking the Keycap off]
Often a small blast of can'ed air will move away any crumbs, a cotton swab (Q-Tip) with some distilled water - moistened not wet (don't use tap water either) will dissolve any sugary junk (soda). And if needed use a cotton swab with some Isopropyl alcohol (85% or better) for anything else and to act as a dryer for the water.