I can't update iso 11 in my ipad 2, the last update is 9.5, some one can tell me how i can update? I try in my itune and don't work

I can't update iso 11 in my ipad 2, the last update is 9.5, some one can tell me how i can update? I try in my itune and don't work

Read more: I can't update iso 11 in my ipad 2, the last update is 9.5, some one can tell me how i can update? I try in my itune and don't work