Add this code to your child theme’s functions.php file:

//Package header label
add_filter( 'wpcfe_multipack_header_label', 'custom_package_header_text' );
function custom_package_header_text(){
	echo 'Your custom label';
}
//Total Volume Weight label
add_filter( 'wpcargo_package_volumetric_label', 'custom_volumetric_weight_label' );
function custom_volumetric_weight_label(){
	echo 'Your custom label';
}
//Total Actual Weight label
add_filter( 'wpcargo_package_actual_weight_label', 'custom_actual_weight_label' );
function custom_actual_weight_label(){
	echo 'Your custom label';
}