iPod 3G update

[John Smith17]

Hi! I have an iPod 3g with iOS 4 installed on it. I'd like to update it to the iOS 5 but there is a problem. There is a jailbreak installed on it. There are a Cydia and SnowBoard app installed on it that do not open at all , the jailbreak is a tethered one. I have downloaded the iOS offline file and I'd want to use it. Because of low Internet speed I can't use Interne. But when I'm going to update it with that file, there appears an error and fails to update. I think at first I should remove the jailbreak but how can I do such? When I click on restore in iTunes it will both want to restore and update via Internet that as I said I can't use Internet. There is not one single button for restore only. Can anyone help me?