You could overwrite the function, but I don’t suggest it. It’s not promised that things won’t be added/removed from the function, and the function name might even change. If you watch updates closely and make adjustments as needed though, it won’t be an issue.
remove_action( 'wp_footer','generate_slideout_navigation', 0 );
add_action( 'wp_footer', 'tu_custom_slideout_nav' );
function tu_custom_slideout_nav() {
$generate_menu_plus_settings = wp_parse_args(
get_option( 'generate_menu_plus_settings', array() ),
generate_menu_plus_get_defaults()
);
if ( 'false' == $generate_menu_plus_settings['slideout_menu'] ) {
return;
}
?>
<nav itemtype="http://schema.org/SiteNavigationElement" itemscope="itemscope" id="generate-slideout-menu" class="main-navigation slideout-navigation">
<div class="inside-navigation grid-container grid-parent">
<?php
do_action( 'generate_inside_slideout_navigation' );
wp_nav_menu(
array(
'theme_location' => 'slideout',
'container' => 'div',
'container_class' => 'main-nav',
'menu_class' => '',
'fallback_cb' => 'generate_slideout_menu_fallback',
'items_wrap' => '<ul id="%1$s" class="%2$s slideout-menu">%3$s</ul>'
)
);
?>
<?php do_action( 'generate_after_slideout_navigation' ); ?>
</div><!-- .inside-navigation -->
</nav><!-- #site-navigation -->
<div class="slideout-overlay" style="display: none;"></div>
<?php
}