@roofboard well basically. You need the cloudron cli as documented here and a git clone of the repo i linked before.
Then in the clone you run cloudron build followed by cloudron update. Along these commands you need to specify the name of your docker image (which requires a docker registry, which could be the docker hub, on Cloudron itself or somewhere else).
You can update an existing installation with a custom fork, and with the right parameters also bring it back to the official app.