By default export file format is .CSV, to change the export file format is very easy by copy and paste this code in your theme function.php file.

// Change the export file format into .xls
add_filter( 'wpcie_export_file_format', function(){
	return "xls";
} );

File format that is accepted to change are:

• xls
• xlt
• xla
• xlw
• csv

To add additional file format in the accepted list, check the sample code on how to.

// Copy and paste this code in your theme function.php file.
// Adding xltx file format in the list
function my_additional_format_list( $format_list ){
	$format_list[] = 'xltx';
	return $format_list;
}
add_filter( 'wpcie_export_file_format_list', 'my_additional_format_list' );