How to replace keyboard in a MacBook Pro 13" early 2011?

If you only lost the key cap element you can replace that alone! Here is one source for the part: The Book Yard - Key Caps for MacBook Pro Unibody.

If you have more damage than just replacing the external key cap parts then you have two choices here. The first is replacing the entire upper case. Here is the IFIXIT guide: MacBook Pro 13" Unibody Early 2011 Upper Case Replacement while still a bit of work it is the quickest way. Here is the needed part: MacBook Pro 13" Unibody (Early 2011/Late 2011/Mid 2012) Upper Case Yes its expensive! but its a lot less work.

OK, what the next option? You'll still need to follow the same IFIXIT guide to take the upper case off. Then, you'll remove the keyboard and replace it. Here is a bunch of eBay listings for the part: Replacement MacBook Pro Keyboards. Yes, it's a lot less, but it's also a lot of work as you'll need to either pop out 50+ snap rivets or screws or a combo of the two. Here's a vid on how its done: Macbook Pro 13" A1278 Keyboard Repair and Replacing. Good Luck!