First, download the appropriate firmware ipsw for your device. You can find all the iPhone firmware download links here. We've also linked the appropriate ones for these steps below.
8.0.0 (6+): iPhone7,1_8.0_12A366_Restore.ipsw
8.0.0 (6): iPhone7,2_8.0_12A365_Restore.ipsw
Connect your iPhone to iTunes via USB and click the Back Up Now button under the Summary tab. It is very important that you back up to your Mac or PC, not iCloud, as iCloud backups won't restore to earlier firmware versions.
Finally, you will need to hold down the Option (Mac) or Shift (Windows) key then click the Check for Updatebutton in iTunes.
Select the ipsw you just downloaded and click Open.
iTunes will now reinstall the iOS 8.0 firmware on your device. Please note the Health app won't work in iOS 8 after these steps; however, it will be fixed in an upcoming iOS 8.0.2 software update. Also these steps won't work forever. Following the release of iOS 8.0.2, Apple will likely stop signing the iOS 8.0 firmware and you will not be able to downgrade after that.
Apple has recognized the issue with iOS 8.0.1 and said, "We are also preparing iOS 8.0.2 with a fix for the issue, and will release it as soon as it's ready in the next few days."