how to upgrade to android 5.0 or higher?

[trucker V]

Ok I have an older LG flex and it has 4.4 on it right now and apparently they are not going to put out an update for this device any longer kinda sucks but oh well. Is there any way I can do it on my own? I just want to have at least 5.0. I have a few apps that require 5.0 or higher and need to do this til I get a new device. The phone is an AT&T device but I'm currently using it on straight talk. (Way less money than AT&T. And the same service) so can this be done? And if so is it a pretty high risk I could damage the device? Thank you in advance to who answers.