To redeem a gift card or promotional code in the Windows Store, follow these steps:
On the Start screen, tap or click Store to open the Windows Store.
Swipe down from the top edge of the screen, and then tap Your account. (If you're using a mouse, right-click, and then click Your account.)
Under Redeem code, enter the code from your gift card or the promotional code, and then tap or click Redeem.
After you've redeemed a gift card or promotional code, when you buy something in the Windows Store, the money in your Microsoft account balance is used first. If you buy something that costs more than what is left in your balance, your payment method will be charged the difference to complete the purchase.