To perform an offline upgrade of a Group Replication group, you remove each member from the group, perform an upgrade of the member and then restart the group as usual. In a multi-primary group you can shutdown the members in any order. In a single-primary group, shutdown each secondary first and then finally the primary. See Section 18.104.22.168, “Upgrading a Group Replication Member” for how to remove members from a group and shutdown MySQL.
Once the group is offline, upgrade all of the members. See Section 2.11.1, “Upgrading MySQL” for how to perform an upgrade. When all members have been upgraded, restart the members.