can disconnect my iphone 5 from itunes after select update to download only

can i disconnect my iphone 5 from itunes

after

i select

             ios6.1.2 to download only not download and install..

and the next thing is i am trying to download ios 6.1.2 from 2 days it will take about 8-to-12 hours remaining to download in itunes and during  download if my pc turns off next time its starts download from beginning again 12 hours to install ..........

i have 1mb internet speed