Note: Please copy and paste this code in functions.php of your current theme.

  function custom_change_shmap_current_marker_url( $image_url ){
    /*
     * Put your new marker url in $image_url variable
     */
    $image_url = "https://www.wpcargo.com/wp-content/uploads/2021/09/wpcargo-marker.png";
    return $image_url;
  }
  add_filter( 'shmap_current_marker_url', 'custom_change_shmap_current_marker_url' );