How to upgrade WordPress Charm
Before updating the Charm you need to backup the database using MySQL charm’s create-backup action.
juju run mysql/leader create-backup
Additional info can be found about backup in the MySQL documentation
Then you can upgrade the WordPress Charm.
juju refresh wordpress-k8s
After upgrading the WordPress Charm you need to update the database schema.
juju run wordpress-k8s/0 update-database