When you're connected to the internet, your Chromebook should automatically check for and download upates. To check for yourself, please do the following:
Click on the time located in the lower right-hand side of the screen
Click on the 'Gear Wheel,' which will open the device's Settings menu
Look for 'About Chrome OS' located on the lower left-hand side of the screen
Click on the option titled, 'Check for Updates.'
After it has checked for updates it will either say 'Google Chrome Is Up To Date' or it will say 'Restart.' If it says Restart please click that button and wait for the Chromebook to shut down, then it will boot back up automatically. At that point you can sign in and you'll be good to go.