Here are the instructions
- Swipe in from the right edge of the screen, and then tap Search. (If you’re using a mouse, point to the upper-right corner of the screen, and then click Search).
- Enter add features in the search box, and then tap or click Settings.
- Tap or click Add features to Windows 8.
- Then tap or click I already have a product key.
- Enter the product key, and then click Next.
- Read the license terms, select the check box to accept the license terms, and then click Add features.