Go back to previous OS version

To do ask you ask you have to wipe the drive and reinstall the compatible OS. You can't go back any further than the OS that was originally installed on the new Mac. It takes a certain amount of "tech savvy".

How to get old versions of macOS - Apple Support

After you have successfully done the task, use Migration Assistant to restore your personal data from TimeMachine. You can also use Setup Assistant during the installation process if your TM drive is available, i.e. connected and can be reached.

TimeMachine does not contain any operating system. It's strictly data, apps and setting.

Simple instructions but understand the below method will REMOVE ALL DATA and there is No Recovery - Period.

All personal Data maybe recovered from a Time Machine Backup

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of ?? would have to be done on a computer that Qualifies to run ??

Downgrading to an older version can only be done if the Computer Qualifies to run the downgrade version of macOS.

Once that is done read on for preparing the Destination computer  >> Only works on Intel Based Apple Computers.

Extra Special Notation regarding the Touch ID equipped Apple Computer.

About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. This applies to macOS 10.14 Mojave and above. 

5A - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled with the GUID Partition Map

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

Command + R (Recovery Panel) will reinstall whatever version is currently installed.

1. Reboot your MacBook into recovery mode (restart and press cmd r)
2. Next, click disk utility, find the erase button, click and wipe the disk
3. exit the disk utility, and press the time machine button. Finally, plug in your time machine drive and restore.

Good luck!