At the moment edits to a scheduled payment is not yet supported. This is something the team is looking at introducing in the near future as we continue to work on improving the application.
In the meantime a workaround would be manually delete and re-add the scheduled payment again once your a transaction that has the updated amount is being pulled through in your transaction list.
Relevant to users on:
Money Dashboard Neon
Not sure which App you are on?
If you are unsure if you are on Money Dashboard Classic or Money Dashboard Neon? You can find that information here