I can successfully hide the back button but can't change image of button.
To hide back button:
NavigationPage.SetHasBackButton(page, bool);
There is Icon attribute but we need to change back button image.
The only way to do this is to override the NavigationBar on each particular OS you have.
NavigationBar
For Android you will need to look at the ToolBar and on iOS is the NavigationBar.
ToolBar
Refer :