You cannot change the amount on a fixed payment. If you wish to send another amount, you need to create a new fixed payment.
You create a fixed payment by initiating a regular bank transfer in the app. When you have entered the amount and the receiver you press “Next”. On the subsequent page you can make it a fixed payment in a given interval and day in the week.
If you have insufficient funds on your account to make the payment, your payment will be declined. You will receive a notification, if the payment has failed.