Is it possible to install Android L on a device that will not get the update in the future? I have Sony Xperia M2, and as you probably know, this phone will not get the new Android L update from Sony.
So is there any way to install the Android L manually?
I don't care to root my device and install a custom ROM which compatible with the new version... but is it actually possible to update my device version?
It is technically possible to install a newer version of Android on that (and probably every other) device.

First you have to make sure the bootloader of your device is unlocked. In doubt just try to unlock it, it won't relock again without notifying you.
Then you flash (in most cases) a custom recovery like CWM, TWRP or Philz.
After that you can install every custom ROM or other zip on your phone.


  The best place for specific Android devices is xda-developers. (imo)
  Most developers meet there and publish their custom ROMs like CyanogenMod as well as modded official firmwares.


For your device there is a big guide on how to unlock your bootloader right on xda: http://forum.xda-developers.com/showthread.php?t=2743694
Installing a custom ROM is then done through your recovery.