Two ways to recover
If an update breaks checkout, a rollback puts the previous version back. A rollforward instead puts a new version in place with a fix.
Going back can be quick, but the old version may no longer work if the update changed stored data. A rollforward can keep other improvements from the update.
Plan for problems
Before a release, teams check how they would restore the old version or publish a fix. Some effects, such as orders already placed, cannot be undone just by restoring old software. The aim is to get a safe, working service back quickly.