You can get the update by installing Microsoft's Zune software to your PC and connecting your phone via a USB cable. Click here to get the Zune app. If you're a Mac user, you need to download the Windows Phone 7 Connector app. The update is not available as a wireless download from AT&T.
Nokia and AT&T are offering all Lumia 900 customers a $100 credit towards their wireless bill because of the glitch. You can also trade in your phone for a new one if you're having trouble installing the software update. Nokia says new Lumia 900s will ship with the software fix.