By default, you can't easily change the operating system on Android devices. On some, it's flat impossible because no one has been willing or able to crack the system. It basically involves rooting the device to gain system level access and making some changes that could brick the device if not performed exactly right.
If it's possible, the method for each is different, even down to the version of the phone. For example, the method to root a carrier unlocked G6 may be different than a Sprint G6. And a Verizon G6 may not have a root exploit discovered yet. This is a hypothetical situation for example purposes. For something as drastic as changing the OS itself, I think it'll also need unlocking the boot loader, which is a whole other ball of wax.
Your husband's best option is to Google what he wants for that specific phone (including the carrier version if applicable), and see what results pop up. The XDA Developers site is usually the best source for these sorts of issues. Be wary of any "one click" type rooting services, as you are doing things the device wasn't designed to do and could be opening yourself to all sorts of privacy issues by letting random services do the work for you.