How to roll back a macOS upgrade
An upgrade is easy to start and awkward to undo. Before you install a new major version, know what going back actually requires on your Mac.
The rule that decides everything
Apple only installs macOS versions it is still signing. Older releases stop being installable over time, and when a version drops out of signing, no amount of local backups will put it back. That single fact means rollback capability expires, and the only reliable preparation is taken before you upgrade.
What to do before upgrading
Make a bootable installer of the version you are currently running. This is the only artefact that survives a signing change, and it is the difference between "I can go back" and "I cannot".
Take a full backup, and verify it restores. A backup you have never restored is a hope, not a backup. Restore one file from it to prove the chain works.
Write down the version number. Not "the latest", the actual number. Restoring means matching exactly, and memory is unreliable under pressure.
What rollback looks like in practice
On Apple Silicon, going back means erasing the machine and reinstalling from the bootable installer or through a second Mac. There is no in-place downgrade, and the erase is not optional.
On Intel, the same shape applies: erase, then install the older system. Firmware that shipped with a newer release cannot be rolled back, which matters if the newer release changed anything you depend on at the firmware level.
Time Machine is not a rollback tool for the operating system. It restores data and can restore a system from a snapshot of the same release, but it will not put an older major version back on a machine running a newer one.
Deciding whether you need this at all
If the machine is not critical and the data is backed up, a failed upgrade costs an evening. If the machine is your income, the preparation above costs an hour and is cheap insurance.
The decision is asymmetric: the preparation is boring and the failure is expensive. Do the boring thing.
A note on timing
Installing a major release in its first week is the highest-variance moment. Waiting for the first dot release removes a large share of the early bugs and costs a few weeks. That trade is obvious for a work machine and routinely ignored for a personal one.
What this page cannot tell you
It cannot tell you whether a specific app breaks, because that depends on the vendor. It cannot tell you whether your version is still signed, because that changes without announcement. Check Apple's current signing status before planning a rollback, and assume the window is shorter than you expect.