How do i change to 7.5 Mango om at&t ?

Can someone please tell me how can i udate my phone to the Windows 7.5 Mango software, I am from India , i got my nokia lumia from a person who has imported it from the USA, its says AT&T, now the phone does not show 3G , whereas it shows 4G, which is not available in here. So when i called up customer care , they are asking me to upload Mango 7.5 but the indian version , so can anyone please help me how i can do that ??

Is this going to be like a hacking thing ?