The Apple iPhone is one of the first gadgets to receive the iOS 7 Beta version and the users could update it from the current iOS version, i.e. the iOS 6 but for that they will have to catch hold of the IPSW file of the iOS 7 Beta, which is not directly available for download unless you are a developer with a paid account in Apple, or have got ways to get the file by yourself for your device.
Update Apple iPhone iOS 6 to iOS 7
It’s currently possible update to iOS 7 without having your iPhone’s UDID registered in a developer account. The only thing you have to do get hold of the ipsw file. As of now we have got it confirmed that the phones where the iOS update to the latest version was possible is the iPhone 5S, iPhone 5 and the iPhone 4S. Check how to download iOS 7 beta.
To update the iPhone to the iOS 7 version you need to connect the iPhone to the computer or Mac, and then open the Apple iTunes and check whether it is at its latest version. If not you would need to update it.
Press and Hold the Shift Key (for windows) / Option Key (for Mac) and while its pressed, click on Update in the iTunes so that it would open the dialog box for you to select the iOS ipsw file.
Select the file, and there would be a dialog box saying that your iPhone would be updated to the latest version and that would be verified with Apple.
Once you click on Update, the process would continue and in a few minutes the iPhone would be updated to the latest version of iOS, and the iTunes would show about the latest version.