Firmware updates are only available in SteelSeries Engine 3, on a Windows PC.
1) Plug in your Stratus XL W&A controller to your PC via the micro USB port located in the battery compartment. You will need a micro-USB to USB cable which is not included with the controller.
2) Download and install the latest version of SteelSeries Engine HERE
It is extremely important to make sure you have the latest version. Here's how to check.
3) Click the red banner below the product that says "CRITICAL UPDATE"
4) Allow the firmware update to run completely and leave the controller plugged in. Once complete, you should get a pop-up dialog that looks like this:
5) Once you complete the process in the dialog box, turn the controller off and on again. At this point it should be paired and ready to go.
Verifying the controller is connected and working: click the Windows icon at the bottom right, type "Controller" and open "Set up USB Game Controllers"
From here, click properties, and under the "Test" tab, pressing buttons and joysticks should show changes on the screen.