Running on netcup is not much different from running on any other Linux host. The main difference is that the host is directly exposed to the Internet, and that it is provisioned with Debian, not Ubuntu, to begin with.
In netcup's customer control panel, Overview, click on "SCP Auto-Login". This gets you into their Server Control Panel.
In netcup's server control panel, choose Media on the left, and the Images tab. Select the Ubuntu 20.04 image and install it, partitioning