Change hamburger icon in master detail navigation xamarin forms

You should use a custom renderer.

In your Android project, like this:

[assembly: ExportRenderer(typeof(CustomIcon.Views.MainPage), typeof(IconNavigationPageRenderer))]
namespace CustomIcon.Droid
{
    public class IconNavigationPageRenderer : MasterDetailPageRenderer
    {
        private static Android.Support.V7.Widget.Toolbar GetToolbar() => (CrossCurrentActivity.Current?.Activity as MainActivity)?.FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);

        protected override void OnLayout(bool changed, int l, int t, int r, int b)
        {
            base.OnLayout(changed, l, t, r, b);
            var toolbar = GetToolbar();
            if (toolbar != null)
            {
                for (var i = 0; i < toolbar.ChildCount; i++)
                {
                    var imageButton = toolbar.GetChildAt(i) as ImageButton;

                    var drawerArrow = imageButton?.Drawable as DrawerArrowDrawable; 
                    if (drawerArrow == null)
                        continue;

                    imageButton.SetImageDrawable(Forms.Context.GetDrawable(Resource.Drawable.newIcon));
                }
            }
        }
    }
}

In your iOS project only use the same icon from you xaml file in your PCL project, like this:

<?xml version="1.0" encoding="utf-8" ?>
<MasterDetailPage  xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:local="clr-namespace:CustomIcon.Views;assembly=CustomIcon"
         Title="MainPage"
         Icon="newIcon.png"
         x:Class="CustomIcon.Views.MainPage">
<MasterDetailPage.Master>
    <local:MasterPage x:Name="masterPage" />
</MasterDetailPage.Master>
<MasterDetailPage.Detail>
    <NavigationPage>
        <x:Arguments>
            <local:Page1 />
        </x:Arguments>
    </NavigationPage>
</MasterDetailPage.Detail>

For more information see my repo on github: https://github.com/wilsonvargas/CustomIconNavigationPage

i applied this tweak and it helped me. now i can see back button also after navigation

protected override void OnLayout(bool changed, int l, int t, int r, int b)
{
    base.OnLayout(changed, l, t, r, b);
    var toolbar = FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
    if (toolbar != null)
    {
        for (var i = 0; i < toolbar.ChildCount; i++)
        {
            var imageButton = toolbar.GetChildAt(i) as ImageButton;
            var drawerArrow = imageButton?.Drawable as DrawerArrowDrawable;
            if (drawerArrow == null)
                continue;
            bool displayBack = false;
            var app = Xamarin.Forms.Application.Current;
            var detailPage = (app.MainPage as MasterDetailPage).Detail;
            var navPageLevel = detailPage.Navigation.NavigationStack.Count;
            if (navPageLevel > 1)
                displayBack = true;
            if (!displayBack)
                ChangeIcon(imageButton, Resource.Drawable.iconMenu2);
            if (displayBack)
                ChangeIcon(imageButton, Resource.Drawable.back1);
        }
    }
}
private void ChangeIcon(ImageButton imageButton, int id)
{
    if (Android.OS.Build.VERSION.SdkInt >= Android.OS.BuildVersionCodes.Lollipop)
        imageButton.SetImageDrawable(Context.GetDrawable(id));
    imageButton.SetImageResource(id);
}