Solution: Although your iOS device syncs every time it connects to your computer, your iTunes software does not check for an iOS update on every sync. iTunes only checks for software updates every few weeks. This way, user updates are spread out over time. The good news is you can manually force an update check from iTunes. Perform these steps.
- Sync your iOS device.
- Click on your iOS device on the left column of iTunes. This will display the sync overview page.
- The second section on this page is named Versions. In this section you will see a Check for Updates button. Click this button and your download and update should start.
Troubleshooting Problems with the Update
Apparently a number of users had problems with their update yesterday, including me. Before you start the process make sure you have installed all the lastest operating system updates. My mac would not perform the update until I installed the new security patch from Apple.
Also, I recommend you reboot your machine. It is amazing how this magically fixes many issues.
The other problem I ran into was time out errors connecting to the update server. Whenever you update your device, it "phones" Apple over the Internet to check some stuff out. (What it is checking, I'm not sure). Anyway, when the server gets overloaded your request to connect fails and you get an error message. If this happens, just wait 30 minutes or so and try your update again.
MacWorld has the details here. The story has a picture of the error message and a little more detail.