Octopi has no update possibility from previous versions does this also go for beta to rc1 or release ?
If not how to update just run the dnf update equivalent for raspian ?
You update raspbian with sudo apt update && sudo apt -y upgrade
But that updates just the base os - not the things that were changed on the Octopi side (like configs or scripts)
I would suggest you backup your Octoprint with the integrated backup, flash the new Octopi image and restore your backup 
I was afraid of that, I must say a bit of a let down for such software to no be able to update. Thanks for the clarification.
It's always been that way and is stated as such on the octopi page