* **This is how I did it. As always, use your own brain as well :)**
Hope I'm not forgetting any steps:

1. get internet access working on your device: https://forum.sailfishos.org/t/release-notes-sauna-4-6-0-11/17815/128
2. get zypper and its dependencies (libaugeas and libreadline) on your device. Some very friendly people here helped me out with getting the rpms.
3. use zypper dup. You will get errors about some packages
4. check for yourself if you need those packages. I did not need those packages so I uninstalled them with rpm (and reinstalled them when the phone was repaired).
5. try zypper dup again, it should work this time.
6. reboot. It should work now.
Afterwards, I was recommended to install and run sfos-upgrade to update from 4.6 to 4.6