Starting with version 2.5.4, the Joomla core is updated with the new component Joomla Update instead of in Extensions/Install/Update.
If Joomla Update does not shows as an installed component the menuitem could be wrong. Run this SQL statement:
UPDATE #__menu SET component_id = (SELECT extension_id FROM #__extensions WHERE name = "com_joomlaupdate") WHERE title = "com_joomlaupdate"
It should update 1 row, if not use the Discover function and install it as a component.
If Joomla Update shows as an installed component but is not in the Admin Menu under Components, you are missing the entry in the database table #__menu.
INSERT INTO `#__menu` (`menutype`, `title`, `alias`, `note`, `path`, `link`, `type`, `published`, `parent_id`, `level`, `component_id`, `ordering`, `checked_out`, `checked_out_time`, `browserNav`, `access`, `img`, `template_style_id`, `params`, `lft`, `rgt`, `home`, `language`, `client_id`) VALUES ('menu', 'com_joomlaupdate', 'Joomla! Update', '', 'Joomla! Update', 'index.php?option=com_joomlaupdate', 'component', 0, 1, 1, (SELECT extension_id FROM #__extensions WHERE `name` = 'com_joomlaupdate'), 0, 0, '0000-00-00 00:00:00', 0, 0, 'class:joomlaupdate', 0, '', 41, 42, 0, '*', 1);
Replace the "#_" of #__menu and #__extensions with the prefix of your database table (System --> Global Configuration --> Server --> Database Tables Prefix).
