Default [Q] what makes android os said that os upgraded?

whats the difference between the old and new android os version in any rom? how can i upgrade it?
example: i have an android phone with version 4.2.2 and i want my phone to use android 4.4 upgrade, but the OEM not provide the updates, so i would update it myself, but i dunno how to make it happen, so i need your help to tell me what must i know before i make it happen

there is so much tutorial made me confused how to do

so i ask you instead who had upgrade your android os yourself ( tell me everything you know )

thanks before for your help