If you have an auto-renewing subscription, you won’t be able to redeem a Gift card/Coupon code while your subscription is active.
If you’ve purchased a one-time Pro subscription that does not auto-renew, you can redeem your gift card/Pro coupon code by heading to the Pro page in your JioSaavn app. Simply click on the "Redeem Now" option (for JioSaavn app version 9.26 and above on Android devices) and continue with the redemption. After a successful completion, the subscription days will be added to your account and you'll be able to enjoy all subscription features for an extended duration.
In case you are on the lower app versions on an Android device, simply head to the 'Activate Pro' section in your JioSaavn app settings. For folks on iOS devices, head to any Android device or the website to redeem your gift card.