You'll be redirected to Roblox's gift cards page. Tap on Redeem, located at the very top of the page. Enter the empty Code field with your Roblox gift card's code, then tap Redeem to finish.

Not to worry, we can help you redeem your gift card, and what you can do / use the gift cards for! How Can I Redeem a ROBLOX Gift Card? ROBLOX Gift Cards come in a selection of two.