The following procedure worked for iOS 6 beta and works for iOS 7 beta (we have confirmed this on an iPhone 5).
Notably, there is always the chance you will brick your device. This is beta software, after all. Proceed with caution; we are not responsible for any issues or damage you may incur.
To perform the iOS 7 upgrade, you'll need an iPhone 4, 4S, or 5 or a fifth generation iPod touch. Apple hasn't provided iPad images as of the time of this writing. Make sure the device is not jailbroken, or restore to pristine non-jailbroken state.
Get the appropriate iOS 7 build (.IPSW) for your device. Select the summary page for your device and press Shift + Check for Update (Windows) or Option + Check for Update (Mac). Do not use the Restore iPhone selection to upgrade to iOS 7 beta or your device will be useless.
Select the appropriate build for your device when the file selection window comes up. After that, your device should update to iOS 7 beta.
The key here is that by doing it with this method you do not have to activate the phone again. If you use the Restore method to update your device, it will be wiped, require activation, and that is where the process will fail.
And that brings us to why we still don't recommend this. If for some reason you need to reactivate your phone (say, because you hard reset it), you will be hosed. You won't be able to activate the device.
(There is also another reason: This is beta software and we've already found a number of bugs).
It is possible to recover, though. You can downgrade to the appropriate iOS 6 build for your device. To do so, since you're device is essentially bricked, place the device in DFU mode.
To put your iPhone in DFU mode:
- Hold down the home and power button for 10 seconds.
- Release the power button while continuing to hold the home button for eight seconds.
We're not going to post links to iOS builds here, but they can be found, we are sure, on Bittorrent and other file sharing sites.