Apple released iOS 4.0.2 in response of jailbreakme pdf exploit.But if you want to downgrade iOS 4.0.2 to 4.0.1 for iPhone 4 and 3GS i’ll give you the complete guideline.
This guide is only for those who saved their SHSH to Cydia Server before the release of iOS 4.0.1 via Cydia “make my life easier” button.
The process are given below.
How to Edit and Save Hosts file in Windows
* Open Notepad with Administrative privileges by clicking Start, All Programs,Accessories, and then right-click Notepad and select to open as Administrator.
* Now browse to (C:/Windows / System32 / drivers / etc) in Notepad and open the hosts file, and add the following entry to the bottom of the file.
22.214.171.124 gs.apple.com <———— Now you navigate to files menu and save the hosts file. By doing this you changed the communication server from Apple to Sauriks Server, because your ECID is on that server. Now you can put your iPhone in DFU mode to restore back to to OS 4.0.1 or lower. How to Edit and Save Hosts file in Mac OS X
* Navigate to Finder, Click to select Go to Folder… from the Go menu and Enter /etc into the input field and click the Go button.
* Now look for hosts file, right click on hosts file and open with Text Edit, and add the following entry to the bottom of the file.
126.96.36.199 gs.apple.com <———— * Now save the file and click on Save As. * Because Text Editor can’t save your system files, here comes the tricky part, in save as windows use the following settings, take a close look at the following image. Uncheck the “ If no extension is provide, use .txt “ and then press the save button. * A host file will be appear on your Desktop, just drag and drop to /etc and follow the onscreen instructions. By doing this you changed the communication server from Apple to Sauriks Server, because your ECID is on that server. Now you can put your iPhone in DFU mode to restore to any of the previous OS. You can also check Steps To Downgrade iOS 4.0.2 to iOS 4.0.1, iOS 4.0(iPhone 4, 3GS, 3G and iPod Touch)
You could always get support from UnlockAlliPhone.com about any Unlock & JailBreak issue of any model of iPhone including iPhone 2G,3G & 3GS and also upcoming iPhone 4. They provide Premium Members Support for life time. Means just pay one time a small fee and you will receive support(real human support – avg response time 2 hour) for all present & future version of iPhone.