I bought a second hand iphone 6s from this chinese new woman at a very good price.

there were no defects and all but when i bought it she placed like a mini chip or a sim whatever and then gave it to me giving me instructions and all she was very helpful because it was my first iphone ever she taught me ways and things. But then she kept on repeating to never update my ios or everything will crash. And so that was like two months ago, i also bought one for my lil bro it was iphone 5s with also the same instructions.

Okay so my lil bro just yesterday woke up with an updated ios11 and its been working pretty fine. Im pretty confused and also torn i wanted to update my ios 10 to 11 but im still scared that the whole phone might just stop working! Help!