So you must have upgraded from windows 8 to windows 10 yes?
If so all you need to do is install windows 8 for now, then download the latest windows 10 iso. You can use the media creation tool to create either an iso (to burn to disk) or a usb install.
Download Windows 10 Once you have created your windows 10 install media on either disk or usb boot into the bios and change the boot order so that whatever you have windows 10 on (either disk or usb) is first in the boot order.
When your installing windows 10 and you get to the section about adding an activation key simply click 'I don't have one' and windows will activate automatically once your online.
This is because your windows 10 licence is stored online