I paid my debit order on the 15th July 2025, on the 19th July I sent email requesting to change my debit order date to last day of the month

I paid my debit order on the 15th July 2025, on the 19th July I sent email requesting to change my debit order date to last day of the month. On the 21st July I called to inform that I would like to change from the 15th to last day of. Debit mandate was sent to me and I accepted the mandate that the debit order will be on the last day of August, now my account is arrears and I've requested to change the debit order date on time.