Woocommerce checks if order is editable or not and it does not allow to edit orders that are processing or completed. But there is a hook to change this. You can simply add this code to functions.php in your theme folder:
function make_orders_editable( $is_editable, $order ) {
// Allow only for admin and moderators
if ( current_user_can( “manage_options” ) ) {
return true;
}
}
add_filter( ‘wc_order_is_editable’,