My iPad 2 is running iOS 6.1.3, but now I want to update to iOS 7. In Settings → General → Software Update, I can only can update to iOS 8.4. iOS 8 destroys your iPad. How can I update my iPad to iOS 7 now, in 2015?
You only ever have two options with iOS - stay on the iOS you're currently on, or update to the latest your device will support.
Apple stops signing all older iOSes within a few days of releasing a new version, meaning that even if you could install it, it would not activate.
As the iPad 2 is supported on iOS 8 [currently 8.4.1] this is your only choice if you wish to update.
If by 'Destroy' you mean your iPad2 locked up (As did mine) when it reboots after iTunes completed the download then fear not! As long as it completed the install ( which seems evident since it rebooted) just remove it from the cable and do a hard reset. i.e. Hold both buttons in for a few seconds and it should restart and now boot with your nice new iOS 9 It will probably show an Apple icon with a cable icon suggesting you had better reconnect to iTunes , Connect back up, then all should be well and iTunes will complete and verify the installation :)