Apple has released iPhone OS 3.0 Software update. Here are the steps how to update iPhone 3.0 OS:
Step 1: Make sure you are using iTunes 8. Connect your iPhone to your computer.
Step 2: When iTunes opens, select your iPhone under Devices in the Source List on the left.
Step 3: In the iPhone Summary pane, click Check for Update.
Step 4: Click Download and Install. Do not disconnect your iPhone until the update has finished.

[via apple]