var collection = (ObservableCollection<MyModel>)this.dg.ItemsSource;
 while (collection.Count > 0)
 {
     collection.RemoveAt(collection.Count - 1);
 }

<Application
    x:Class="TelerikWinUIApp7.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:localPrimitives="using:Telerik.UI.Xaml.Controls.Grid.Primitives"
     xmlns:primitivesCommon="using:Telerik.UI.Xaml.Controls.Primitives.Common"
    xmlns:core="using:Telerik.Core"
    xmlns:local="using:TelerikWinUIApp7">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
                <!-- Other merged dictionaries here -->
                <ResourceDictionary Source="ms-appx:///Telerik.WinUI.Controls/Themes/Generic.xaml"/>
            </ResourceDictionary.MergedDictionaries>
            <!-- Other app resources here -->
            <Style TargetType="localPrimitives:DataGridEditRow">
                <Setter Property="CommitButtonStyle">
                    <Setter.Value>
                        <Style TargetType="primitivesCommon:InlineButton" BasedOn="{StaticResource EditRowButtonBaseStyle}">
                            <Setter Property="BorderThickness" Value="1 1 0 1" />
                            <Setter Property="ContentTemplate">
                                <Setter.Value>
                                    <DataTemplate>
                                        <StackPanel Orientation="Horizontal">
                                            <TextBlock Text="&#xe73e;"
                                               FontFamily="{ThemeResource SymbolThemeFontFamily}"
                                               VerticalAlignment="Center"
                                               Margin="15 0 0 0"/>
                                            <TextBlock core:LocalizationManager.Key="Save"
                                                       core:LocalizationManager.PropertyName="Text" Margin="8 0 15 0"/>
                                        </StackPanel>
                                    </DataTemplate>
                                </Setter.Value>
                            </Setter>
                        </Style>
                    </Setter.Value>
                </Setter>
                <Setter Property="CancelButtonStyle">
                    <Setter.Value>
                        <Style TargetType="primitivesCommon:InlineButton" BasedOn="{StaticResource EditRowButtonBaseStyle}">
                            <Setter Property="BorderThickness" Value="0 1 1 1" />
                            <Setter Property="ContentTemplate">
                                <Setter.Value>
                                    <DataTemplate>
                                        <StackPanel Orientation="Horizontal">
                                            <TextBlock Text="&#xe711;"
                                               FontFamily="{ThemeResource SymbolThemeFontFamily}"
                                               VerticalAlignment="Center"
                                               Margin="15 0 8 0"/>
                                            <TextBlock core:LocalizationManager.Key="Cancel"
                                                       core:LocalizationManager.PropertyName="Text" Margin="0 0 15 0" VerticalAlignment="Center"/>
                                        </StackPanel>
                                    </DataTemplate>
                                </Setter.Value>
                            </Setter>
                        </Style>
                    </Setter.Value>
                </Setter>
            </Style>
        </ResourceDictionary>
    </Application.Resources>
</Application>