How to re-redeem iTunes card?

You need an iTunes account for that. Here are the steps to redeem your gift cards:

Download iTunes onto your computer if you do not already have it.

Open iTunes, and click on "iTunes Store" on the left side of the screen.

Go to the "Quick Links" window on the right side of the screen, and click "Redeem."

Enter the code printed on the back of your iTunes card into the space provided, and click "Redeem." This will credit your iTunes account with the amount of money on the card.