This article will help you.
Unfortunately it doesn't seem to be possible to just convert a Windows 7 key to a Windows 10 key, you'll have to install Windows 7 first, upgrade it and then do a clean install of Windows 10 from there. It's pretty complicated, but that's the Microsoft way.
Wow. Thanks for the link, I'll check it out, but the microsoft way is really not the easiest of ways. :P
They always seem to try to make it as complicated as possible.
Be careful when you set up your dual-boot, when you install Windows it'll erase your existing bootloader, so you need have a live-usb handy to re-install grub (or whatever it is you use). Otherwise you won't be able to boot into Linux anymore.
They still do that shit? Man, it feels like 2004. -_-