You can do a google search. There are sites out there have host the actual iOS files and they walk you though on doing a manual update. I know i had to look up some stuff a while back because my cousin's iTouch was like iOS 6 which needed like iTunes 11 or something like that but their iMac is so old that it doesn't support the newer versions of Mac OS which are required for the new version of iTunes so I had tried to find a way to down grade his iTouch and found a site for that.
i want an expert person's reply,
ok i managed to download my desired ios .its 6.1.3, so now if i update it in itune menu by holding the SHIFT button and click restore and select my this downloaded ipsw file.
After installation whn my phone requires activation will apple sign my this ios, and i will have no issues that my soft ware is not signed and im stuck eith it,
Any help will appreacited coz im new in this iphone world