If you installed a beta version of iOS 8 on your iPhone or iPad and now find yourself thinking the bugs and crashes are more than you bargained for, you may want to return to iOS 7 again. Luckily, the downgrade process is fairly straight forward and simple. However, it does come with a few caveats...
What you need to downgrade from iOS 8 beta to iOS 7.1.1
In order to downgrade to iOS 7.1.1 from iOS 8, you'll only need the following:
A PC or Mac running the most recent version of iTunes — 11.2.2 is the current version
Warning: You may lose all your data
Before beginning, it's very important to understand that you will lose all your data from iOS 8. You can not restore from an iOS 8 backup on iOS 7. If you have an older iCloud or iTunes backup from iOS 7, you can restore to that. However, if you only have an iOS 8 backup now, you won't be able to use it, so keep that in mind.
How to downgrade from an iOS 8 beta to iOS 7.1.1 official
Turn your iPhone or iPad running iOS 8 off.
Hold down the Home button and plug it into your computer. Keep holding the Home button until you see a screen telling you to connect to iTunes.
iTunes should show you a popup saying that there is an iPhone or iPad that is in Recovery Mode and needs to be restored before you can use it. Click OK.
Click on Restore iPhone... — or Restore iPad if that's what you're using.
A popup will appear asking if you're sure you'd like to restore the iPhone or iPad in question. Click on Restore and Update.
iTunes will then download the most recent official firmware and install it — which in this case will be iOS 7.1.1.
That should be it. Once your iPhone or iPad reboots you should be back on iOS 7.1.1 again.
Keep in mind that you can't install to a previous version of iOS 7 since 7.1.1 is the only one Apple is currently signing. This is the reason that this process currently works. It will cease to work once the iOS 8 official comes out this fall, so keep that in mind if you think you aren't going to want to keep iOS 8.