It's done with the Bose Connect application. Tap the options icon in the upper right hand corner. Slide the settings menu up until you see and then tap on "Product Info". Everything including firmware is on the next screen. (The top of the settings screen will tell you if you have the most up to date firmware. If not, it will ask you to download and install).
The app is the best way but if you can not install it on your mobile device (or don't have a mobile device), there is a web site as well, https://btu.bose.com/