Unfortunately it's no long possible to downgrade from iOS 8 to iOS 7 - Apple is no longer signing the iOS 7 images so won't be able to manually install the older images.
Step 1: Go to Settings > General > Usage > Manage Storage.
Step 2: Select “iOS 8.0.1.” Because it is over 1GB, it is probably towards the top of your list.
Step 3: Tap Delete Update, and tap Delete Update again to confirm.
This will effectively remove the downloaded update from your device and prevent you from accidentally installing it.