How can I fix the system?

[AC Question]

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 The out-of-date system is causing my apps to run slow, it's causing my camera to have really bad quality.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.

 

[B. Diddy]

Welcome to Android Central! You may very well have a clone or knockoff, because according to the original Android Central review and to Wikipedia, the S6 originally shipped with 5.0.2, not 5.0.1. So that certainly could explain why the performance is so poor. It also means that updating the system probably won't improve much, and updating will likely be challenging since it doesn't seem to be genuine.

Install and run CPU-Z. What processor does it identify? If it's a MediaTek, it's clearly a knockoff.

 

[IAmSixNine]

After trying what B. Diddy suggest you can also download Samsung Keis to your computer and try to use that to update the phone.