Changing the default tab position

<?php

// Copy from here to your (child) themes functions.php

function my_custom_product_tabs_order( $tabs ) {
	
	// Double check to make sure the default tabs exist and are not removed at some point.
	if ( isset( $tabs['description'] ) ) {
		$tabs['description']['priority'] = 30;
	}
	
	if ( isset( $tabs['additional_information'] ) ) {
		$tabs['additional_information']['priority'] = 20;
	}
	
	if ( isset( $tabs['reviews'] ) ) {
		$tabs['reviews']['priority'] = 10;
	}
	
	return $tabs;
}
add_filter( 'woocommerce_product_tabs', 'my_custom_product_tabs_order' );