How to unlock an iphone 3GS?

My boyfriend iPhone r and it's on o2 but he wants to unlock it to put his original sim card in it, how can he do that?

Http://www.idownloadblog.com/2010/08/01/how-to-unlock-your-iphone-3g-or-iphone-3gs-ios-4-with-ultrasn0w/

It's not difficult, relax. Depending on the iOS which is presently on the device, there are software unlocks available online which correspond with the iOS installed on the phone.

To Unlock I-phone 3GS from United Kingdom(UK) O2 network lock get the Remote Unlock service from the site http://www.wickedunlock.com/o2-uk-apple-iphone-unlocking-service/rs11wp10/ and Unlock it easily.

First of all, you have to jailbreak your phone using a program such as Redsnow or QuickPwn. This allows you to run applications on your iPhone that have not been approved by Apple. This is essential to unlocking your iPhone 3GS because the applications (e.g. Cydia) used in the unlocking process are not Apple approved.

After that, you have to
- Go to "Settings" >> General
- Press on "Network" >> OFF "Enable 3G"
- Go to "Menu" >> Press the "Cydia" icon on your iPhone home screen.
- Press "Manage" when the app loads >> Source >> Press "Add"
- Enter in "repo666.ultrasn0w.com" (at Enter Cydia / APT URL) >> Add Source
- Go back to the Cydia sources menu and you will see the Ultrasnow package
- Click on it to download and wait as it is installed
- Turn off the iPhone.
- Use the paper clip to press the SIM tray release button on the top of the phone and remove the SIM card
- Place your new SIN card in the tray and close it
- Turn your phone on
- Your new network provider's details should appear on the screen.

If they do not, please check that the SIM card is correctly placed in the iPhone tray.

Your phone is now unlocked to any network.