To Activate Windows 10 OS, you need to purchase a license. You can purchase a license from the official Microsoft store or from the store nearby you.
Open the Settings app on your Windows 10 device
After that click on the Update and Security option and then open Activation option from the left sidebar.
Click on the change Product Key option from that page and enter a new product key you have.
Then follow the on-screen instruction and Windows 10 will be activated.
You can also activate your windows with a digital license. You have to purchase a license from your Microsoft account and then log in to your Windows with that account. Your windows will be activated and you will feel happy.
Get more information, visit here: Install Windows 10