Publish on iOS rejected

Hi,
publish on iOS is rejected for this reason:

Regarding 4.6 guideline, we continue to find that your app includes user-selectable icons but does not provide a way to change the icons within the app.

To resolve this issue, it would be appropriate to revise your app to provide the users an option to manually change the icons within the binary. Please ensure that your app’s icons can only be changed at the user’s request, are relevant to the content and functionality of your app, and can be reverted back to your app’s original icon.

What they have in mind? I can’t found any idea how to resolve in UE?