Our system only allows one subscription at a time. You can use the gift certificate on your account when your current subscription expires.
You can disable the auto-renew on your subscription, then claim the gift certificate when the subscription ends.
If you subscribed on an iOS device such as an iPhone you can manage your auto-renew option through the iTunes account settings on your iPhone. Please see this support article for details: http://support.apple.com/kb/ht4098
If you subscribed on an Android device, please open the Google Play Store app and then select "My Apps" and look for the Subscriptions tab, or go to the Google Play Store website, click on the Settings gear and go to "My Orders." Please see this support article for details: https://support.google.com/googleplay/answer/7018481
If you subscribed through our website, please let us know the email address on your account and we can cancel the subscription for you.