Update objects using entity framework 5 + wpf

I try to get an entity updated using wpf+entity framework 5. I have a datagrid which looks like this:

<DataGrid x:Name="activityDataGrid" 
                                  AutoGenerateColumns="False"
                                  EnableRowVirtualization="True"
                                  ItemsSource="{Binding Source={StaticResource activityViewSource}}"
                                  RowDetailsVisibilityMode="VisibleWhenSelected"
                                  IsReadOnly="false"
                                  SelectedItem="ID" 
                                  SelectionChanged="activityDataGrid_SelectionChanged" 
                                  CellEditEnding="activityDataGrid_CellEditEnding" 
                                  RowEditEnding="activityDataGrid_RowEditEnding">
                            <DataGrid.Columns>
                                <DataGridTextColumn x:Name="col_activityDate" Header="Datum" Binding="{Binding DATE_, StringFormat=\{0:d\}}" Width="Auto"/>
                                <DataGridTextColumn x:Name="col_activityDescription" Binding="{Binding DESCRIPTION}" Header="Tätigkeit" Width="Auto" />
                                <DataGridTextColumn x:Name="col_activityDuration" Binding="{Binding DURATION}" Header="Dauer" Width="Auto" />
                                <!--<DataGridTextColumn x:Name="iDColumn" Binding="{Binding ID}" Header="ID"/>-->
                                <DataGridTextColumn x:Name="col_activityIsBillable" Binding="{Binding IS_BILLABLE}" Header="verrechenbar" Width="Auto" />
                                <DataGridTextColumn x:Name="col_activityIsDrive" Binding="{Binding IS_DRIVE}" Header="Anfahrt" Width="Auto" />
                                <DataGridTextColumn x:Name="col_activity_task" Binding="{Binding TASK_ID}" Header="Aufgabe" Width="Auto" />
                                <DataGridTextColumn x:Name="col_activityWorker" Binding="{Binding WORKER.USERNAME}" Header="User" Width="*"/>
                            </DataGrid.Columns>
                        </DataGrid>

c#:

private void activityDataGrid_RowEditEnding(object sender, DataGridRowEditEndingEventArgs e)
    {
        //how to update the object??
        db.SaveChanges();
    }

Thanks in advance!