I did it. Twice. Not a fun thing to do. You are not suppose to do it, so there is no apple supported procedure. The best you can do is reinstall from scratch, maybe using a bootable USB with El Capitan, and restore files and apps from Time Machine. I tried restoring the whole OS using Time Machine, and it never worked.
At the end, I went back to Sierra, and I'm staying here for the moment.
BTW, which apps are not working for you? I found that some of them need a reinstall, even though Time Machine restores them, the configuration gets corrupted and they crash. Happened to me with Mail, Webex and with Colormuki Display. I was about to post a feedback about Colormunki to the beta team, then I tried just uninstalling and reinstalling it from scratch. It works flawlessly.
With Webex was even harder. I had to download a Webex Removal tool to clean every configuration about it, reinstalled, and working ever since. No issues whatsoever.
So, i'd recommend you reinstall your problematic apps before rolling back.