It’s going to be in the install directory on your machine (location may vary). In my case, Users>Shared>UnrealEngine>4.13>Engine>DerivedDataCache.
With the Launcher and editor closed, delete that folder. It (should) rebuild on Launch. If not, you can create an empty folder and name it “DerivedDataCache.” -This is the most common solution when the Editor is stuck at 35% on a Mac. However, while this is a good place to start, it may be something else.