Hi Ralf
A Windows 10 licence allows you to install Windows 10 on just one PC or Mac at a time . .
If you do want to install Windows 10 on that PC, you would need to purchase a Windows 10 licence, then install Windows 10 from a USB stick as explained below:
Click this link:
https://www.microsoft.com/en-us/software-downlo...
to download the Media Creation Tool (Click on Download Tool Now), with that you can download the latest Windows 10 ISO (Select Create Installation Media for Another PC), you can create a bootable USB flash drive (min 8GB) using that tool
Then, Boot your PC from the Installation Media you just created (change Boot Order in your BIOS) to begin installing Windows 10