The component Joomla! Update allows you to easily update the CMS directly from the administration. To access this component, select Component (1) → Joomla! Update (2).
Joomla! Update Component
The page you'll be taken to will vary along with the version of your CMS, whether it's up-to-date or not:
You have to update Joomla!
1 - You can clear the cache to search for new updates.
2 - Configure the components' parameters.
If you have to update Joomla!, all you have to do is to click the Install the Update button (3). Joomla! will then upload the update patch and apply it to your website.
It is important to always keep your site (and your installed third-party extensions) up-to-date. Updates are fixing bugs and security vulnerabilities, so do install them.
Caution, this component only deals with the updates of Joomla! itself (and of its native extensions). If you installed third-party extensions, you have to update them via Extensions → Extension Manager → Update.
It's also important that you back up your entire site before installing an update so that you can go back and recover files if problems arise.
In the admin control panel, you will find 2 icons notifying you when new updates are available: