1) Go to
https://ipsw.me/ a) Select iPhone
b) Scroll down and select iPhone 2G
c) Select any version of iOS 3 you want to restore to.
d) Click download.
2) Connect your iPhone 2G to your computer.
3) Put your iPhone 2G into DFU recovery mode by:
a) Pressing and holding the power button and home button down for 10 seconds.
b) Then, release the power button while still holding the home button down.
c) You can release the home button once iTunes detects the iPhone 2G is in DFU mode (if not, try again).
4) Then in iTunes, hold the Shift key (for Windows users), or the Option key (for MacOS users), and press the ‘Restore iPhone’ button.
5) Select the IPSW (iOS) file you downloaded.
Your iPhone should now be restoring (this may take some time).
Once your iPhone has finished, you should be on whatever version you restored to!
Written and edited by Jonathan Yeung