If you are using Windows 10, it requires you to activate the license in order to get the most out of it. If you are a retail user and have a single license Product Key of Windows 10, you can't use the same key on another system unless you deactivate the key on the first device.
In case you didn't yet enter a product key during the installation, or used a generic key that you want to change, or want to upgrade your installation with a new Windows 10 product key, here are simple steps to follow.