I am using v.1.46.9 of the docker image of nextcloudpi.
In the ncp dashboard and in nc the version installed shows 20.0.4. On github it is stated that nc 23 ist supported. Why is my version stuck on 20 and how do I update?
Yep, you have to upgrade major version by major version.
But, you should be able to pull the latest docker image and rebuild your container on current version.
Just be sure to make a backup first of both your data and your ncp settings under :4443 or ncp-config on terminal.