How To Update Your Chromebook
If your Chromebook is connected to wifi, it will make updates automatically. You will know there is an update available if you click the menu at the lower right-hand corner of the screen and see "Update Available." Click "Restart to update" and your computer will restart and finish updating the system.
If you want to manually look for updates, click on the Settings gear in that same menu.
In Settings, click the three lines, and then click "About Chrome OS" at the bottom of the menu.
You will see text that says "Check for updates." Click on that.
If your Chromebook finds an update, it will download automatically. When the update is done downloading, you can click "Restart" to restart your Chromebook and finish updating.
Images from howtogeek.com