UPDATE ipod on?? where is it!?

Ok, i plug in my ipod. and this time its full , but i deleteed some songs, so when i plug it in again. it should update...delete the songs off the ipod whitch i already deleted, and it should put the new ones on i have put in itunes..BUT all the suddden it dosetn update when i plug it in... Wheres the ( Update ipod) on on itunes. hwo do i TELL it to update cus it wont...