Why wont my lg g3 update to marsmallow

[AC Question]

I just got an unlocked phone off eBay and it won't update to marshmallow and it's stuck in 5.0.2 and won't update I factory reset it and it still won't update. Plz help

 

[belodion]

Are you talking about OTA updates? When you look in Settings, About Phone, Software Update, what does it say?