Will replacing the upper case fix my problem?

Unfortunately, the answer is maybe. Replacing the upper case is not an easy repair. Have a look at this guide if you are thinking of doing it. Read through all the steps so you get an idea of what you are getting into. Whether this would fix the problem depends on how much liquid got into the guts of your machine and how far it went. You won't know that answer until you open the case. If the liquid is only in the keys and track pad you may be in luck and the repair should fix the problems. On the other hand, if you find the logic board and other components covered in sticky residue you may have corrosion happening in the connectors between parts. If you want to give this repair a try read this guide too. If liquid got all the way down into the little connectors, turning on and using your computer before doing a thorough cleaning may be encouraging more corrosion. So if you think you are up for the repair do it sooner rather than later and leave the computer off until you can do the cleaning.