Where is the auto update for Joomla?

From Joomla! Documentation

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Starting with version 2.5.4, the auto update for Joomla is now located in Components→Joomla! Update, as shown here:

Version-2-5-5-faq-screenshot.png

Extensions are still updated using the Extension Manager (Extensions→Extension Manager). If you click on the Joomla! Update icon in the Control Panel, it will link you to the Joomla! Update component automatically.