@marcus Right, there was a move away from installation scripts in Debian/Ubuntu to a package approach and these got step-by-step upgrade writeups. I’m not aware of a Centos package in the works. However, if you had a source code install you could just pull the updated code in the course of the upgrade processes.
I’m not sure if this approach would be promoted by everyone here, but it’s one way. In the end, generally speaking, make backups, get the v3 files in place, and then add back in your backed up config, avatars, server keys and other pieces/parts for your specific install, run db migrations, do cleanup, etc. and the app will recognize these changes as you start working with it again.
You could instead also copy the current passbolt directory to the side and start with a v3 git install but my concern in recommending that was not knowing your current setup. If there is a problem it might be hard to tell what is causing it since we are upgrading and changing install methods at the same time. If you are able to confirm a functional source code install based on a previous version tag, then pulling new changes should be pretty straightforward.
Sorry there is a not already a clear step-by-step but hopefully some of this helps you. Post back with any technical questions, and someone in the forum will help. There’s a good number of Centos users so others feel free to chime in.