Hi Jamie 
You should be able to mange Umbraco Cloud subscriptions in the Cloud Portal:
https://www.s1.umbraco.io/
Note that this is a separate portal outside the Umbraco Backoffice.
More information on payments in Umbraco Cloud can be found in the Umbraco Documentation:
https://docs.umbraco.com/umbraco-cloud/set-up/new-shop