Hopefully, you followed my advice to create a Time Machine backup before installing the Sierra beta.
HOW TO BACKUP MAC TO ICLOUD OS X EL CAPITAN INSTALL
And if you took the bold step to install the Sierra beta right over El Capitan, then you'll need to roll back the clock and use a Time Machine backup that predates your installation of the Sierra beta. If you followed my earlier advice to install the Sierra beta on a partition, then it's easy to erase and remove that partition. If you installed the public beta of MacOS Sierra to give it a spin but have now satisfied your curiosity, you can return to OS X El Capitan while you await Sierra's final release this fall.