This has been a persistent problem with Google’s Android operating system, as releasing a major version for it never happens simultaneously across all devices on the platform. Sometimes, manufacturers of specific handsets also like to make their own unique changes to the OS, requiring them to then release their own updates that account for their modifications as well.
This has created a messy situation where users sometimes resort to manually flashing their devices in order to circumvent the long waiting periods, though this carries the risk of “bricking” the phone, or in other words making it unusable unless it’s repaired by a technician.
Manual flashing is a mostly safe procedure when the standard process is followed, but a small mistake can sometimes cost the user hundreds of dollars. But when the next official update is nowhere in sight and could take months to come out on the market, there’s usually no better option than to plug a USB cable into the device and flash a custom ROM onto it manually. At least Android is open enough to allow that.
Lucky for Sprint users, my Lollipop update screwed up my AT&T S4 forcing me to upgrade. I wish I never accepted the update.