First of you need the following:
- The 4.2.1 ipsw which you can find in the following folder
- ~/Library/iTunes/iPhone Software Updates/
In here you should see a file called iPhone3,1_4.2.1_8C148_Restore.ipsw (iPhone 4). If you had another iPhone like the 3GS you would see something similar but it would start with iPhone2,1
- If you want to restore your phone from a backup, that is easy too as long as you haven't deleted old backups. I saw a post that was explaining what to edit in the latest backup (in case you did it while in 4.3) which consisted of editing the backup's info.plist file. This consist of editing 2 strings to state the version and build so you don't get the can't restore from back up message. Even though I tried this I then received a message stating the back up was corrupt so I just chose an earlier backup I had that was a 4.2 backup. You can also just choose to set up as a new iPhone. If you want to try editing the latest backup you can find the file in the following directory and edit the 2 string listed below.
- ~/Library/Application Support/MobileSync/Backup/
- <key>Build Version</key><string>8C148</string>
- <key>Product Version</key><string>4.2</string>
- Turn off the iPhone (you can hold down the Sleep button, or use Sleep + Home, and release as soon as the screen shuts off).
- Hold down the Home button while connecting to your computer with iTunes already open and ready for a connection.
- iTunes will prompt you to perform a software restore.
Your iPhone will obviously be wiped new and be reset as a result and you wil have a working iPhone again.
- Click the "Restore" button while holding down the option "alt" key and select the ipsw file.