Currently the only way to launch UEFN is by having the entire game closed, and opening through “Launch Creative” on the dropdown menu in the Epic Games Launcher.
It’d be so much easier if the ability to launch the editor was also built into the game at some point (assuming this would be planned already for launch) would save a lot of time, especially when dealing with crashes and having to wait for the “Game Running” text to close before you can relaunch.