To update the amount of a subscription using the API, you’ll need to cancel the existing subscription by sending a POST request to the Subscriptions endpoint.
This won’t affect the customer’s Direct Debit mandate, so you can create a new subscription for the updated amount with a second POST request.
It’s also worth noting that cancelling a subscription will not cancel any payments that have already been created. If any payments are pending_submission, you can cancel them by sending a POST request to the Payments endpoint.
Similarly, if you need to change the charge date of a subscription, you’ll need to cancel the initial subscription and create a new one with the updated charge date.