Interim Files: Once the downgrade has complete and you’re back to El Capitan, Mavericks, etc, you’ll likely want to manually copy back over any of the interim files you saved from Sierra. In case it wasn’t obvious, that is why Time Machine is called Time Machine, by the way, since it effectively lets a user step their OS and files back in time if need be. In this example, that means the Mac is now back on MacOS X El Capitan 10.11.6, and macOS Sierra 10.12 is entirely removed from the Mac as the restore process effectively rolled back the Mac to before Sierra was installed.
Once the Mac has finished restoring from Time Machine, the prior version of Mac OS will automatically boot with all of the data backed up from that time period. Let the “Restoring” process complete, this can take anywhere from 30 minutes to several hours, depending on the size of the hard disk, the size of the backup, the speed of the computer, and the speed of the backup drive.Confirm that you wish to erase the target drive (“Macintosh HD” or otherwise) and restore it from the chosen Time Machine backup – this is irreversible, the drive will be formatted and erased and restored from the backup – click on “Continue”.Optional: if FileVault is enabled, click on “Unlock” and authenticate to disable FileVault encryption before you can use the Restore function.At “Select a Destination”, choose the destination Mac drive to restore to, typically this is “Macintosh HD” then click on “Restore”.
How to Downgrade from macOS Sierra with Time Machine If you skip that, you’ll lose the interim data. I personally get around this by creating a folder on the Time Machine volume and manually dragging and dropping important new documents into it, then just copy them back over to the restored Mac, but some users rely on iCloud Drive, DropBox, or other services. * Remember, a side effect of rolling back with Time Machine is that any interim data will be missing unless you manually back up the files yourself before beginning the process (for example, if you restored from January 1 but it’s now January 15, you would lose files created or modified between those two dates unless you manually back them up before the restore). A manual backup (separate from Time Machine) of any interim documents or data created between the time of the Time Machine backup and now*.