Lg g4 imei.info says its AT&T 810 but software settings and behind battery cover its 815 model?

[Smhyder]

Dear members admins plz guide me in this situation that my phone lg g4 software and behind battery cover its model shows up as h815 but imei info reads it as AT&T 810 I have heating & battery drain issues as well lte band 1800-3 in my country its not picking even though h815 specs shows that band is supported and whenever I try to update to MM instead of rom update apps update menu pops up.
My question is how can I update it to marshmallow 6... should I use flash tools or lg up and should I use AT&T 810 stock rom or 815 and should I while updating completely wipe and format option or just update option ?
Regards.