Etihad Airways Flight Date Change Not Working in App

I have a return ticket with Etihad Airways from Munich to Abu Dhabi and then Abu Dhabi to Islamabad. My current return flight is booked for 31 August at 11:25 from Munich. I want to change this return to 1 September at 11:25 from Munich. When I logged into the Etihad app and tried to change my booking, the system showed that the ticket cannot be changed and the change option was not working. Because of this technical limitation, I was unable to make the date change I need, even though I am trying to manage my booking through the official Etihad channel. My booking reference is ANWNTX. I request that you allow me to change my return flight from 31 August 11:25 to 1 September 11:25 from Munich through the app or online system, without forcing me to go through extra steps. I kindly ask you to resolve this issue as soon as possible and inform me about the process.