In general, smartphone updates are based on Phone Software Operating System and / or Phone Hardware Software (firmware). Usually it is very difficult to update Phone Software OS alone. For example, if you have an Android phone with Gingerbread, even though your phone hardware is capable to run the next Android version (4.x), I don’t think there is a way to load the latest capable Android OS for your phone to your phone without doing root / custom ROM. So, you have to depend on the updates roll out by OEM / Mobile Operators.
In case of Windows Phone, there is an option to upgrade phone OS to the latest released OS if your phone hardware supports it. If there are any new features which is newly enabled in OS but not enabled in your phone firmware then it will not work but the user interface / new programs / features related to existing hardware will work fine. For example, in Nokia Lumia 520 phone, FM radio chip was available in hardware but the firmware and OS didn’t support it. When the OS allowed FM radio app, unless Nokia Lumia team releases a firmware update to enable the hardware in Lumia 520, you can not make use of FM radio option even though you have installed latest OS with FM Radio feature.
http://www.wpcentral.com/how-sign-windows-phone-81-preview-developers this webpage article explains how to get the latest Windows Phone 8.1 software OS update to your Windows Phone 8 hardware (when the OS updates actually release). This page also clarifies some of the FAQ related to this OS alone update without firmware.
http://dev.windowsphone.com/en-us/featured/devpreview has more details.