When an update is available, users with the permission to update Craft will see a badge in the CP next to the Utilities navigation item in the sidebar. Click on Utilities and then choose Updates. You can also use the Updates widget on the Control Panel dashboard, which is installed by default.
This section displays both Craft CMS updates and plugin updates. Each update its own Update button. Clicking that will initiate Craft’s self-updating process.
You can run all of the updates (Craft, all plugin updates available) using the Update All button at the top left of the Updates page.