I have upgraded to MacOs. How is it possible to go back to my earlier Operating System?

Wipe and restore from your last backup from the previous environment. Otherwise, the downgrade is a manual project, starting from a wiped environment and importing what files you can from your backups. This gets tricky, as some apps can alter file formats and contents in ways that may not be compatible with older versions of the apps from earlier releases.

Restore a Time Machine backup or bootable clone you made of it, or reinstall it onto a blank volume.

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