If you redeem a gift subscription on your account while on an active monthly, 6-month, or annual subscription it is likely saved to your account as a credit.
You can use this credit by clicking here, then by going to "Manage" under the "My Subscription" tab and it will prompt you to update to either a 6 month or 12 months with your unused credit on your account.
If you run into any issues processing this on your account, please message our Customer Support Team by clicking on the chat box in the bottom right of your screen!