There are a number of software components in the Juju universe that can be upgraded. Please visit the page dedicated to the task you're interested in.
- Upgrading a machine series
This affects the series (release) of the underlying operating system of a Juju machine, including a controller.
- Upgrading models
This affects the version of Juju running on a Juju machine in the form of machine agents and unit agents.
- Upgrading applications
This affects the version of an already deployed charm.
- Upgrading the client
This affects the version of the Juju client.
There is no inherent reason to upgrade components at the same time or to upgrade components in a specific order. What often dictates an upgrade is a desired feature in a new version of one component. This may, either before or after, necessitate the upgrade of another component.