If you are running Rancher in Single node mode. You don’t have to delete it container but we don’t want that container to be started by accident after the upgrade. I normally recommend keeping for a couple of days just in case you need to roll back.
It is important to note that single node mode is no longer supported in Rancher v2.6+ for production installs and as it’s really only designed for POC or lab environments.
We do have a migration path which can be found at Migrating a Rancher Single Node to a HA Installation