You can update your billing details at any time, although the changes will not take effect until the next billing cycle.
- Click on the your profile icon
- Click on Update Details to change your payment details
You can cancel the subscription by simply clicking on the Cancel button.
If you are paying for your vPOP PRO subscription within the iOS App then your payments are managed by Apple.
Apple - About canceling a subscription
Apple - Change, add or remove Apple ID payment methods