There are 2 parts of the charm can can be upgraded. The charm itself, and the
snap-store-proxy.snap that is manged by the charm.
If the snap is installed from the snapstore, and the instance on which it is installed, can reach the snapstore, it will update itself automatically.
If the snap is installed as a resource, then upgrading means attaching the newer version of the snap as a resource. In this case, also the core20.snap needs to be updated manually.
Upgrading the charm is done via juju, it is simply a matter of running the juju command to upgrade applications
juju refresh snap-store-proxy-charm