Hello @Omar Houseiny! I'd advise you to give us a call at 0800 0402. My colleagues can help you change your bankaccount number. 

Meanwhile, have you received a reminder to pay your invoice? Then there are several possibilities to pay your outstanding invoice. You can always transfer the money directly from your bank: KPN Thuis NL41INGB0000467598 or KPN Mobiel NL83INGB0000325513. Both addressed to KPN B.V.. Always mention your KPN accountnumber. You can find your KPN accountnumber on the invoices.