Can I change the amount of an already created payment order?

No. If there is an error when the amount is entered, you have two options: cancel the order if it is still in progress or duplicate it to create a new payment order with the new desired amount.

Note that if you want to recreate a new payment order, the first payment order will not be disabled by the gateway. If needed, you can disable it yourself.

See chapters Can I cancel a payment order? or Can I duplicate a payment order?