Where to use menu_link_save
You don't need to put in the update function. 
It can go anywhere in your module just Remember to clear the cache after you update the module so you can see it.

If you're adding new menu items in a module update you should put:

cache_clear_all('*', 'cache_menu', TRUE);


In the mymodule_update_7000 function and it will clear the menu cache automatically when you update the module.

Additionally: You might be better using hook_menu for the menu items in your module.

something like this:

function mymodule_menu() {
  $items['guarantee/validto'] = array(
    'title' =&gt; 'meeeeeeeeeeeeeeeeeeeeeeeeeeeeeenu',
    'type' =&gt; MENU_NORMAL_ITEM,
    'weight' =&gt; 0,
    'menu_name' =&gt; 'main-menu',
    'page callback'   =&gt; 'drupal_goto',
    'page arguments' =&gt; array('guarantee/validto'),
  );
  return $items;
}


It can go anywhere you like in your mymodule.module file but generally goes near the top. Again, you don't need to put in the update function but remember to clear the cache after.