I have an iphone 6 but overtime I tried to update it it goes wrong and does not update or even turn on

Hello, is there anyone who can tell me whats wrong? I have an iphone 6 but overtime I try to update (this is the second time it happens) something goes wrong and in the screen it only appeared the itunes sign like making a connection and then says it can update, don't even start anymore and I can not use it. Can anyone tell me how to fix this problem??