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There is no automatic migration of major/minor versions of Kubernetes. You need to update your clusters manually or press the Upgrade button in the Dashboard.
Before updating a cluster you should be aware of the potential errors this might cause. The following video will dive into a Kubernetes outage in production that Monzo experienced, its causes and effects, and the architectural and operational lessons learned.
It is therefore recommended to first update your test cluster and validate it before performing changes on a productive environment.