Revert your Mac to a previous macOS version
If you use Time Machine to back up your Mac, you can easily revert to a previous version of macOS if you experience trouble after installing an update. To select a Time Machine backup from an SMB server, your Mac must have macOS Sierra or later installed.
Important: Reverting erases any changes you made to the files on your Mac since you installed the new version of macOS. To save new or revised files, copy them onto a different disk or back them up before you follow these instructions.
Print these instructions.
Click the Share button
in the Help window, then choose Print.On your Mac, choose Apple menu
> Restart. After your Mac restarts (some Mac computers play a startup sound), press and hold the Command and R keys until the Apple logo appears, then release the keys.Select Restore from a Time Machine Backup, then click Continue.
Select your Time Machine backup disk.
Select the Time Machine backup you want to restore. To restore your computer to the state it was in before you installed the new version of macOS, choose the most recent backup that occurred before you installed the new version.
Follow the onscreen instructions.
If you backed up files as described in the Important message above, you can restore them now. If you backed them up using Time Machine, click the Time Machine icon 
in the menu bar, choose Enter Time Machine, then retrieve your files.
If you reinstall a previous version of macOS on a Mac or partition, new backups will use that version of macOS. If your backups are stored on a Time Capsule or other network storage device, existing backups are retained; they aren’t overwritten by the new backups.