From APN Changer and Data Sim Guide
There are three methods of changing the GPRS APN settings, one through browser, one through a cydia app and other through file editing.
Change iPhone APN Settings Through Safari Browser
1. Connect your iPhone to some wifi/wireless network which can give your iPhone Internet access.
2. From your iPhone Safari browser, open the website : m.apnchanger.org
3. Choose Custom APN
4. Type the APN settings for your carrier,click Install.
5. Save the profile and re-start your iPhone. Your phone should now be working with the GPRS settings.
Enable Cellular Data Network Settings On iPhone Using APN Editing App
For this you can download the app called “APN Editing”
Enable Cellular Data Network Settings Though File Editing
1. Download and Install OpenSSH app from Cedia
2. Download and Install WinSCP client for Windows . Run the client and connect to your iPhone using SFTP over the wifi network. The Host Name will be IP address visible in
“Settings > General > Wifi > <Network Name>”
username will be “root”
password will be “alpine” for iPhone 3Gs
3. Browse to the following location on the WinSCP client
“/System/Library/Carrier Bundles/<carrier name>” and copy the “carrier.plist” file from this folder. carrier name is the folder name that depends on which carrier you are using. For example: Airtel India, so the path is:
4. Download and run pledit software and open the file “carrier.plist” downloaded in above step with pledit editor
and paste the following code just after the “<dict>” tag in this file:
Save the file after making changes.
5. Copy the Saved file back to the same folder in the iPhone. Restart the iPhone and the settings will be enabled.