Assuming you did NOT use the mod launcher:
For any mod, especially overhaul mods, it's best to install in a separate folder than your main steam game folder. BUT, if you *have* installed into your main steam game folder, the easiest method to go back to vanilla would be to:
rename the current "7 Days to Die" folder to something like "7 Days to Die - Darkness Falls", and then that'll keep DF installed in that game folder that you can run the .exe directly from to play the mod, and then go into steam, right click the game, choose Properties, choose Local Files, then click "Verify Integrity of Game Files" to re-acquire the vanilla files in a regular steam folder.