Update my iPad 2 to iOS 7 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.