2 Answers

Instructions

1
Open Apple iTunes.

2
Connect your iPhone to your computer using the Apple iPhone USB cable. The iPhone appears in iTunes under the "Devices" menu.

3

Click "iPhone" under "Devices" in the left menu.

4
Click "Check for Update." ITunes will notify you if your iPhone has any pending updates or if it is fully updated.

5
Accept the terms and agreements of the new update and follow the prompts to finish your update. The iPhone may restart during this process.

Read more: http://www.ehow.com/how_8163914_update-iphone-3g.html#ixzz2bGHXIiyG

Automatically:
1. Download iTunes 10 from Apple.com .
2. Connect your device to the computer.
3. Now you will be shown a message when iTunes come up.
4. Click the >Download and Install > button to update iPhone 3G/3GS/4 .
Manually:
1. Download iTunes 10 from the above link.
2. Download iOS firmware for your iPhone from Tipsneeded.com .
3. Connect iPhone 3G/3GS/4 to PC.
4. Choose your device from the left side list.
5. Press the Shift key(in Windows) or the option key(in Mac OS X), click on the Restore button.
6. Now browse to the iOS firmware update and select it. But make sure whether your Iphone get unlocked using permanent unlocking solution. Because if you unlocked your Iphone using temporary unlocking solution it will get locked to its network again when you update or upgrade it. If not unlock your Iphone using remote unlocking service from the site Theunlockarena.com ,which is a permanent unlocking solution. Then you can upgrade IOS on your Iphone3G it won't get locked again.