Look for the following section of FeatureDataForm default control template. I have added a "CancelButton" on the left side of "CommitButton", i.e.
<Button x:Name="CancelButton" Content="Cancel" Grid.Column="0" HorizontalAlignment="Right" HorizontalContentAlignment="Center" Margin="0,0,12,0" MinWidth="75" MinHeight="22" Style="{TemplateBinding CommitButtonStyle}" Visibility="Visible" VerticalAlignment="Center" VerticalContentAlignment="Center"/>
...
<Grid Height="166" Width="246">
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<ScrollViewer Background="{x:Null}" IsTabStop="False" Padding="{TemplateBinding Padding}" Style="{StaticResource CustomScrollViewerStyle}" VerticalScrollBarVisibility="Auto">
<ContentPresenter x:Name="ContentPresenter" Margin="12,12,6,12"/>
</ScrollViewer>
<Grid Margin="0,8,0,8" Grid.Row="1">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Button x:Name="CancelButton" Content="Cancel" Grid.Column="0" HorizontalAlignment="Right" HorizontalContentAlignment="Center" Margin="0,0,12,0" MinWidth="75" MinHeight="22" Style="{TemplateBinding CommitButtonStyle}" Visibility="Visible" VerticalAlignment="Center" VerticalContentAlignment="Center"/>
<Button x:Name="CommitButton" Content="{TemplateBinding CommitButtonContent}" Grid.Column="1" HorizontalAlignment="Right" HorizontalContentAlignment="Center" Margin="0,0,12,0" MinWidth="75" MinHeight="22" Style="{TemplateBinding CommitButtonStyle}" Visibility="Collapsed" VerticalAlignment="Center" VerticalContentAlignment="Center"/>
</Grid>
</Grid>...