Lg g2 not registered ?? :( Help me pls :'(

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Hai, so I want to update my phone because i still have the 1st update from AT&T but I'm not on AT&T anymore because it was too expensive for me, I am on MetroPCS right now but every time I try to update it tells me that I can't because I'm not registered yet. Can somebody help me ? I was told to put an AT&T sim card but i don't have one and nobody i know has one... I'm on Cloudy G3 right now, but i'm planning to go back to stock. How can I update it without having to be on AT&T ? I can't find the answer anywhere .. I'm asking this question because my phone is only getting like 2 1/2 - 3h on screen on time with really light usage.. I was told that LG G2 was the king of battery life.. Not really impressed gotta say .. Please someone help me :'(