This code snippet changes the map marker in the User Locations extension.

um_user_locations_marker_data.php

<?php

/**
 * Change the map marker title and image.
 */
add_action( 'wp_footer', function () {
  ?><script type="text/javascript">
    wp.hooks.addFilter('um_user_locations_marker_data', 'um_user_locations', function (marker_data, hash, userdata) {

      // Change the map marker title.
      marker_data.title = userdata.display_name;

      // Replace the map marker image.
      if ( 'object' === typeof marker_data.icon ) {
        marker_data.icon.url = 'http://website.domain/wp-content/plugins/ultimate-member/assets/img/default_avatar.jpg?um_avatar_marker=1';
      }

      return marker_data;
    }, 20);
  </script><?php
}, 20 );

This gist is a part of the article User Locations - Hooks

You can add this code to the functions.php file in the active theme directory. Skip the opening <?php tag.