Can I upgrade RAM on an early 2014 Macbook Pro with Retina display?
No, you cannot add more RAM.

Open up Activity monitor, located in the Utilities folder of the Applications folder (or just search using Spotlight)

/Applications/Utilities/Activity Monitor

Go to the memory tab and check out the memory pressure and swap used. If the memory pressure is too high (yellow/red graph) you can figure out which applications are using the most memory by going to the View menu and choosing Columns -> Real Memory. Sort applications by the newly added Real Memory column and you will see which applications/processes are taking the most memory and slowing/freezing your computer.