A lot of people on IOS seems having problem to upgrade to iOS5 having issue, one common “Unknown Error -23″. it is possible of corrupted IOS downloaded, solution which is to update it manually.
1. Download the iOS5 manually. See iOS 5 Now Available:
iPhone 4 GSM
2 For Mac users, copy the iOS5 file to “~/Library/iTunes/iPhone Software Updates“.
Win XP users “\Documents and Settings\username\Application Data\Apple Computer\iTunes\iPhone Software Update”
Win Vista / 7 users “\Users\username\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates“
3 Go back to iTunes and hit “Update”.
Hope it works