Not sure that you can Choagy.
See here:
http://www.redmondpie.com/downgrade-5.0.1-to-4.3.3-4.3.5-5.0-ios-on-iphone-4-3gs-ipad-2-ipad-1-ipod-touch-guide/
Quote:
Before you continue, though, keep in mind that you can only downgrade if you have previously backed up SHSH blobs for whichever version of iOS (4.x and below only) you wish to downgrade to. This is because of certain security checks used by Apple to ensure people stick to the latest version of iOS.
Why does he want to downgrade exactly?
Miles 
He wants to downgrade as a few weeks ago the phone went muckin' fental and he lost all contacts/mp3/pictures and when he reset it/restored it the phone started to backup . The backup has taken about 16 hours so far so as can be expected he is a bit peeved
to say the least. He reckons that the earlier OS was more stable and fancied going back to it 
Choagy FFCUK The SPL 
Hmmm, that's a bit unfortunate.
On the plus side, the new iCloud stuff (as part of iOS5) means that contacts, pictures etc. can be automatically synced with 'the cloud' and also a PC, so you don't lose them if you lose your phone. So I do think it's worth the upgrade.
16 hours is far too long to do a backup, so something's going amiss there. My iPhone 4 backups take approx 30 seconds!
That is rather too long, I'd have cancelled it long before then and tried again. ![]()
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