Upgrading is a common procedure, as you pick up bug fixes within the same MySQL release series or significant features between major MySQL releases. You perform this procedure first on some test systems to make sure everything works smoothly, and then on the production systems.
Downgrading is less common. It is typically performed because of a compatibility or performance issue that occurs on a production system that was not uncovered during initial upgrade verification on test systems.
Downgrade from MySQL 8.0 to MySQL 5.7 (or from a MySQL 8.0 release to a previous MySQL 8.0 release) is not supported. The only supported alternative is to restore a backup taken before upgrading. It is therefore imperative that you backup your data before starting the upgrade process.