I have iPad A1396.I can't install new software.

Your iPad is an iPad 2.

It can be only updated to iOS 9.3.5, but seriously, quite a few iPad 2 users have reported into these forums complaining that iOS 9 had seriously slowed their iPad 2 performance and none of these iPad 2 users have been happy about that since.

If your iPad 2 is running fine on iOS 7, I would just leave it there.

Plus, Apple has been engaging in undocumented feature ommisions that started hapenning with iOS 8.

If you really want to take the chance and update your iPad to iOS 9.3.5, you must do it by connecting your iPad to a computer running the latest the version of iTunes.

So, Good Luck to You!

Why can you not install newer software. The iPad 2, which is the version you identify, should support up to iOS 9.3.5. When you connect the device to iTunes and attempt to update, what happens? See this support document for update instructions. Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support