Not all devices will get all updates, and some will get them later than others. Only way to update if there is no official update is to install a custom ROM,which can cause issues with the phone like no data working, or calls not working or being dropped.
If you want a device that will get updates for a while, get a higher end phone, usually the lower spec models are not updated as fast or as often and at some point them stop offering updates for them at all.