Make sure that you have downloaded the latest Philips Hue app and set up a portal account linked to your hue bridge.
Go to the "Settings" > "Software update" and follow the onscreen instructions. The Philips Hue app will tell you if you are up to date or if a software update is being downloaded.
A notification will appear when new software is available for your Bridge or Lamps.
You will receive a notification in your Philips Hue app to upgrade your system. This notification is only for updating the Hue bridge firmware.