So in System > Software Management > Boot Locations
You'll see the drives as you have noted in Image list.
You'll not the Unused boot location will have a "link" on it. click that, it'll take you to another page, where you get the option to "Activate" the location and REBOOT! Be warned, make sure you are in the right big-ip and you are ready for the down time / maintenance window. So this will start the reboot process! I've done it once when i thought i was in a different box, and rebooted something i shouldn't have!