This is likely due to viewing the page prior to joining the guild, Try again 10 minutes after joining and you should see it in the list.

This is possible after the billing period, So if your subscription is Monthly you will be able to change it after setting it in the next month and if yearly in the next year. For yearly Subscribers you can put in a support request to change this how ever do note It'll treated as if it were monthly so if you changed it within the last month already you will have to wait.