I found this in a comment on the iTunes Store from Fred Fassberger, and thought it would be super helpful for me later on. I can't tell you how many times I've updated an app, found it was buggy, and wished I could go back to the prior version until the bugs are fixed. So, I thought, maybe this could help others too.

HTH,
Maya-G

INSTRUCTIONS TO REVERT iOS APP BACK TO PRIOR VERSION
(This applies to ANY app in the Apple app store, btw)

IMPORTANT: Immediately after doing ANY app update you MUST retrieve your prior app versions from your Finder's trash. Yes, every single time you update an app in iTunes the old version is removed in iTunes and simply placed in your Finder's trash. It's still usable so DON'T DELETE IT!

TO SAVE PRIOR APP VERSION:

1. Create a folder somewhere on your computer to archive your old app versions in case you need to revert back. Suggestion: label the folder "App Archive Folder".
2. Update (your app) in iTunes. If you updated it on your iPhone or iPod Touch then do a sync and iTunes will store that last version of the app (in your Finder's trash).
3. Drag prior version of the app from your Finder's trash and place it in your handy-dandy new App Archive Folder.
4. Sync your device. You now have the most recent version of (the app) on your device and the prior version in your App Archive Folder.

TO REVERT BACK TO PRIOR APP VERSION:

1. On your device delete (the app): hold the app icon until it starts to jiggle then press the X. This will delete the app and all app data, Yes! Press the Home button to stop the jiggling.
2. Back on your computer, drag the old version of (the app) from your App Archive Folder to the "Library" section of your iTunes window (upper left).
3. You'll be asked if you want to replace the current (newer) version of the app. Yes, confirm.
4. Connect your device to your computer.
5. Select your device on iTunes side pane.
6. Select the APPS tab.
7. Sort by name. Select (the app) and check it.
8. Sync your device again.

You have now reverted back to your prior app version.

Remember, this can be done with ANY app...just always remember to drag the older versions of updated apps from your Finder's trash to your own App Archive Folder and do the steps above to revert back.