In order to redeem your product code follow the instructions:

Log in to your Epic Games account
Visit this website here
Click on Get Started to begin the redemption process.
Enter your PIN code. Once you finish entering your code click on Next.
Select the platform that you want to use your V-Bucks on and click on Next. If the platform you want to use isn’t available be sure you are logged into the correct Epic Games account.
Review the details and make sure the Epic Account, device, and new balance are correct then click on Confirm.
You have successfully redeemed your V-Bucks card on your Epic Games account. Drop in to Fortnite to spend your V-Bucks!