I purchased a Galaxy S6 from a Pawn Shop, brand new, $360. The system is old, though. It is Android 5.0.1. But when I try to update it, it says that the SIM is not registered to update the server and tells me to reboot. I've tried rebooting it, but it doesn't work. I was told I could have a fake but it seems exactly like the real ones, I've researched it. The only thing that's different is the system. I have tried every way I can to fix it, but it won't work. The out-of-date system is causing my apps to run slow, it's causing my camera to have really bad quality. Please HELP. Thanks.