There are different ways to upgrade MySQL 5.7 to 8.0. You can use the upgrade in-place or even create a replication slave in the new version, so you can promote it later.
But before upgrading, step 0 must be backing up your data. The backup should include all the databases including the system databases. So, if there is any issue, you can rollback asap.
Another option, depending on the available resources, can be creating a cascade replication MySQL 5.7 → MySQL 8.0 → MySQL 5.7, so after promoting the new version, if something went wrong, you can promote the slave node with the old version back. But it could be dangerous if there was some issue with the data, so the backup is a must before it.
For any method to be used, it’s necessary a test environment to verify that the application is working without any issue using the new MySQL 8.0 version.