Before updating, you should back up all your data and settings. For the beta, Apple advises that you should back up to iTunes specifically (rather than to iCloud) and then archive the backup.
Enter Recovery mode by holding the Power and Home buttons until your iPhone or iPad turns off, then continue holding the Home button. This can take a while, so don't give up after a few seconds.
When it says 'Connect to iTunes', do exactly that - plug it into your Mac or PC and open up iTunes.
iTunes will tell you that it's detected an iPhone or iPad in Recovery Mode, and that this needs to be restored - click OK to agree to this, and then Restore iPhone or Restore iPad (as appropriate).
Finally, choose the option to Restore and Update. iTunes will download the latest version of iOS 10 and install it on your Apple device.
Once iTunes has reinstalled a fresh version of iOS 10, you have a choice of either restoring from an old backup (remember, your iOS 11 backup won't work) or setting it up as a brand new iPhone. Simply select your preferred option, follow the on-screen prompts and voila! You're back on good ol' iOS 10.
I am aware that there are only a few firmwares right now that are still being signed by apple. I downgraded from 11 to ios 10.2.3. I'm not going to tell you which ones are available, you can search for yourself. Anyways, hopefully this works. Have any problems? Don't hesitate to reply.