After you have installed a copy of Windows 8 on your computer, you may at some time have to activate it if you haven’t done so already – or you may want to change your product key and upgrade your Windows 8 to a higher version.
Windows Activation is the initial process by which a Windows running on a PC is determined to be properly licensed and genuine, and it’s really quick and easy. It is different from Registration, in the sense that, Activation is the process of ensuring that your copy of Genuine Windows is used according to the Microsoft Software License Terms, whereas registration is the process of entering information to sign up for product support, tools and tips, and other product benefits.
Activate Windows 8
Well, Microsoft has simplified this process and all you need to do is, open Control Panel and navigate to System and Security > Action Center > Windows Activation. If you have not entered your Product License Key and/or not activated your copy of Window 8, you will see a note to this effect in the Action Center. Click on the link to enter your License Key and Activate it. Make sure you are connected to the Internet. You can get more information here on how to activate any version of Windows.
Once activated, you may wish to view the Licensing Status and Activation ID of your Windows OS with slmgr.vbs.
Upgrade Windows 8
If you have already entered your key and activated it, but want to add more features to your Windows 8 by upgrading to a higher version like Windows 8 Pro or Windows 8 Enterprise edition, type slui.exe in the search bar and hit Enter to open the Windows Activation dialog box.
Clicking on the Add features to your edition button will give you two options.
Clicking on the I want to buy a product key online will open your browser and take to a web page where you will be able to buy a genuine Windows 8 key from Microsoft.
If you have already purchased a key, you will have to click on I already have a product key link. This will open up a box, where you can enter your new product key and click Next.
The computer will reboot once done. This will upgrade your Windows 8 and add new features to it.
Check this post if you want to upgrade Windows 8 to Windows 8.1.
This post on the difference between Windows 8 and Windows 8 Pro may also interest you.