The following steps using PC iTunes over USB cable enabled me to revert to Opera Mini browser 6.0.1 after a quick, disappointing test of 6.5.1 on an iPad running iOS 5.0.1, and should work the same way on an iPod.
The following presumes the iOS device is normally using iCloud backup, and also that Opera Link was previously used to save bookmarks and Speed Dial targets. Other apps may have other customization steps akin to Opera Link.
- Disable backup to iCloud on device (I overlooked this precautionary step in my test).
- Disable backup to iCloud on device within PC iTunes (if needed).
- Backup device via USB cable to PC iTunes.
- Update app on iOS device over wireless.
- Test updated app on iOS device.
12/7/11: If you are going to test the app for an extended time, DO NOT leave the app on the device when syncing the device with PC iTunes before completing testing, because the sync will wipe out your old version of the app in PC iTunes. If you are saving any data with the new version of the app, you're going to have to give it all up should you need to delete the app for this scenario. I overlooked this when I was testing 6.5.1 on my iPod, so I lost my 6.0.1 version in PC iTunes, but fortunately 6.5.1 turned out to be a superior replacement.
- If you prefer the older version of your app, delete the updated app from iOS device, then manually copy the older version back to the iOS device within PC iTunes.
- Use Opera Link to restore bookmarks and Speed Dial targets (the latter will get up-to-date page images).
- Disable Opera Link (unless you normally keep it enabled).
- Move the (restored) Opera Mini icon to your desired Home Screen location.
- Re-enable backup to iCloud on device.
- Re-enable backup to iCloud on device within PC iTunes if needed.
Remember that PC iTunes can save (specific versions of) apps even if they later get removed from the App Store. Some apps are removed because they are replaced by a newer version, and some get totally removed (often enough without explanation). I haven't tested, but something like the above likely works from iCloud backup, except perhaps for apps removed from the App Store.
6/25/12: The following (or something close to it) is behavior I observed a while back during testing, which is quite logical but caught me by surprise:
If you have multiple iOS devices syncing with the same PC's iTunes, be careful with any app which is a Universal Binary (same app runs on both iPod/iPhone and iPad size screens, making full usage of the two respective screen sizes) which you use on all devices. If you update it on one device and sync that device with the PC's iTunes, the other device(s) will also get that app updated to the newer version when it syncs with that PC's iTunes.