How do I do this? I need to make another folder the default folder as my ssd is getting crazy full.
Do you mean the folder for the projects, the engine or the marketplace stuff?
Marketplace –> Go to your C - UnrealEngine - Launcher - VaultCache - choose a folder - move it to another drive - delete the folder in the VaultCache
https://answers.unrealengine/questions/70347/choose-download-location-in-market-place.html 