Warning : Once you update iPhone 4 or iPhone 3GS to iOS 5 beta there will be no way to downgrade your baseband and you will also lose your unlock. If you want to downgrafe iOS 5 to iOS 4.3.3 or older firmware then you must have saved SHSH blobs (you can find from Cydia for which firmware your SHSH blobs are saved.
Downgrade iOS 5 to 4.3.3, 4.3.2, 4.3.1, 4.2.1, 4.1 On iPhone 4, 3GS, iPod Touch 4G, iPad [How To] :
1) Download iOS 4.3.3, 4.3.2, 4.3.1, 4.3, 4.2.1, 4.1 IPSW file and iTunes here and save it on desktop.
2) Connect your iDevice running iOS 5 beta to PC or Mac and switch off.
3) Now follow below instructions to put your iPhone, iPod Touch or iPad in DFU Mode :
Press and hold Home and Power Button together for 10 seconds, after 10 seconds release power button & keep holding home button for 25 seconds.
4) Once your iDevice is in DFU mode start iTunes and click on OK button iDevice in recovery mode alert.
5) Now if you downgrading to iOS 4.3.3 then go to step 8.
6) Those who want to downgrade to iOS 4.3.2 or older firmware will need to point Apple server ip to Saurik’s Cydia server.
For Windows :
Navigate to C:WindowsSystem32driversetc and locate hosts file.
For Max OS X :
Go to /etc folder
Now Open hosts file in any text editor and add following line
Now, save hosts file.
7) In iTunes select your iDevice and press Shift (Windows)/Option (Mac) key board key and click on Restore button.
iTunes will prompt for location of IPSW file, use browse and select iOS IPSW file and click on OK to start estore process.
9) It will take a few moments, now your iDevice will reboot with the older firmware instead of iOS 5.
Now let iTunes restore iOS 4.2.1 on your iPhone,iPad, or iPod Touch. During the restore process you may see unknown error messages (1013, 1004 etc), ignore these errors.
At this point your iDevice may be stuck in recovery mode, showing Connect to iTunes. To get rid of recovery mode, download TinyUmbrella and click on Exit Recovery option.