Build the package on your PC with deploy/make-release.sh 1.2.0 (or .ps1 on Windows), then upload it here.
Every client sees "Update available" in his admin panel and installs it with one click (or automatically if you enable auto-update for him).
Every client server needs this (it's in the install config of each client). It can only check licenses, not create them.
Back up data/signing-key.pem on this server. If it's lost, every client must be re-configured.