Giz has some very detailed instructions describing how to install iOS 5 onto your iPhone (and only your iPhone). The trick depends on an activation workaround based on clicking widgets to get past the main lock screen.
We’ll leave it up to you to download and install iOS 5 (search for torrents, basically, and Option+Restore your iPhone in iTunes) but here’s how to crack the activation:
Activate iOS 5
1. Triple click the home button. This will activate the Voice Over.
2. Triple click the home button and Emergency Call will appear.
3. Click on Emergency Call and, while it’s switching, swipe with your three fingers down.
4. The Notification Center will appear!
5. Click on the Weather widget. The Weather app will load.
6. Click on the home button to exit to the iPhone’s springboard.