If you have a computer already activated on Windows 10 and want to be able to use this process in the future, see Microsoft's steps on linking your Microsoft account. Windows 10 made it a lot easier to re-activate Windows 10, thanks to the ability to link your Windows 10 licenses to your Microsoft account. How to activate Windows 10 with a digital license
If you don't have a product key, you can also go from the Activation page directly to the Windows Store to buy a product key. You might do this if you're switching editions of Windows 10. You can follow these steps to activate a Windows 10 license even if you already have one active on your system. (Note: you'll need to be online to activate.)
Step two: Press the Windows key, then go to Settings > Update & Security > Activation (or type 'activation' into the search bar).Image Credit: TechRadar (Image credit: TechRadar)
