You can center align the text of the Expander Header in Xamarin.Forms SfExpander by using the HorizontalTextAlignment and VerticalTextAlignment properties.
You can refer the following article.
XAML
Set HorizontalTextAlignment and VerticalTextAlignment of the header label to Center.
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:syncfusion="clr-namespace:Syncfusion.XForms.Expander;assembly=Syncfusion.Expander.XForms"
x:Class="ExpanderXamarin.MainPage">
<ContentPage.Content>
<ScrollView BackgroundColor="#EDF2F5">
<StackLayout>
<syncfusion:SfExpander>
<syncfusion:SfExpander.Header>
<Grid HeightRequest="50">
<Label Text="Veg Pizza" TextColor="#495F6E" VerticalTextAlignment="Center" HorizontalTextAlignment="Center"/>
</Grid>
</syncfusion:SfExpander.Header>
<syncfusion:SfExpander.Content>
<Grid Padding="10,10,10,10" BackgroundColor="#FFFFFF">
<Label BackgroundColor="#FFFFFF" HeightRequest="50" Text="Veg pizza is prepared with the items that meet vegetarian standards by not including any meat or animal tissue products." TextColor="#303030" VerticalTextAlignment="Center"/>
</Grid>
</syncfusion:SfExpander.Content>
</syncfusion:SfExpander>
<syncfusion:SfExpander>
<syncfusion:SfExpander.Header>
<Grid HeightRequest="50">
<Label Text="Non-veg Pizza" TextColor="#495F6E" VerticalTextAlignment="Center" HorizontalTextAlignment="Center"/>
</Grid>
</syncfusion:SfExpander.Header>
<syncfusion:SfExpander.Content>
<Grid Padding="10,10,10,10" BackgroundColor="#FFFFFF">
<Label Text="Non-veg pizza is prepared by including the meat and animal tissue products." HeightRequest="50" TextColor="#303030" VerticalTextAlignment="Center"/>
</Grid>
</syncfusion:SfExpander.Content>
</syncfusion:SfExpander>
</StackLayout>
</ScrollView>
</ContentPage.Content>
</ContentPage>
Output