Xbox gold - extend subscription

Hi. I'm new to Xbox. I have bought a 12 mth subscription for my children but want to extend to 24 mth now, not wait til 12mth expires. From my account summary I can see the option to Manage my existing subscription, but options only include change payment
type, turn off recurring billing, cancel subscription etc. Where do I find the option to extend the subscription to 24 months and buy the extra 12 mths now?

Thank you in advance for your help.

:)