You can customize the .NET MAUI SfExpander header icon in the SfExpander.Header elements using the converter.

XAML

Binding the IsExpanded property to show different icons to expand and collapse. Set HeaderIconPosition to None to hide the default header icon.

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:syncfusion="clr-namespace:Syncfusion.Maui.Expander;assembly=Syncfusion.Maui.Expander"   
             xmlns:local="clr-namespace:ExpanderMAUI"
             x:Class="ExpanderMAUI.MainPage">
    
    <ContentPage.Resources>
        <ResourceDictionary>
            <local:ExpanderIconConverter x:Key="ExpanderIconConverter"/>
        </ResourceDictionary>
    </ContentPage.Resources>
    
    <ScrollView BackgroundColor="#EDF2F5" Padding="0,30,0,0">
        <StackLayout>
            <syncfusion:SfExpander x:Name="expander1" HeaderIconPosition="None">
                <syncfusion:SfExpander.Header>
                    <Grid HeightRequest="50">
                        <Label TextColor="#495F6E" Text="Veg Pizza" Padding="5,0,0,0" />
                        <Image Margin="10" HorizontalOptions="End" Source="{Binding IsExpanded,Source={x:Reference expander1},Converter={StaticResource ExpanderIconConverter}}" HeightRequest="20" WidthRequest="20"/>
                    </Grid>
                </syncfusion:SfExpander.Header>
                <syncfusion:SfExpander.Content>
                    <Grid Padding="10,10,10,10">
                        <Label TextColor="#303030" Text="Veg pizza is prepared with the items that meet vegetarian standards by not including any meat or animal tissue products." HeightRequest="50" VerticalTextAlignment="Center" />
                    </Grid>
                </syncfusion:SfExpander.Content>
            </syncfusion:SfExpander>


            <syncfusion:SfExpander x:Name="expander2" HeaderIconPosition="None">
                <syncfusion:SfExpander.Header>
                    <Grid HeightRequest="50">
                        <Label TextColor="#495F6E" Text="Non-veg Pizza" Padding="5,0,0,0" />
                        <Image Margin="10" HorizontalOptions="End" Source="{Binding IsExpanded,Source={x:Reference expander2},Converter={StaticResource ExpanderIconConverter}}" HeightRequest="20" WidthRequest="20"/>
                    </Grid>
                </syncfusion:SfExpander.Header>
                <syncfusion:SfExpander.Content>
                    <Grid Padding="10,10,10,10">
                        <Label TextColor="#303030" Text="Non-veg pizza is prepared by including the meat and animal tissue products." HeightRequest="50" VerticalTextAlignment="Center" />
                    </Grid>
                </syncfusion:SfExpander.Content>
            </syncfusion:SfExpander>
        </StackLayout>
    </ScrollView>
</ContentPage>

C#

Return image based on the IsExpanded property value.

public class ExpanderIconConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if ((bool)value)
            return "arrowdown.png";
        else
            return "arrowup.png";
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}