Log into your Customer Control Panel, go to your Products page, click on the magnifying glass icon to open the server you want to transfer, go to Cancellation/Kündigung and click "Add product to account transfer", if needed, it will generate an closing invoice for you, after this has been processed the server will is enabled for account transfer and you will see a transfer code. Give this code to the new owner, as soon as they add it in their account, the server will be gone from your account.
Make sure to format the server drive and install a fresh OS before transferring it.
ManuelK already explained the change of ownership process in detail and the netcup help center page is here:
Change of Ownership
EN: Yes - change of ownership of two servers to another netcup customer is possible.
DE: Ja - der Inhaberwechsel von zwei Servern zu eine anderen netcup Kunden ist möglich. (https://www.netcup.com/de/helpcenter/…/inhaberwechsel)
