In order to handle a high ball to your backhand or forehand in tennis, you need to adjust in two ways: prepare higher and hit more across the ball.
This video shows you Roger Federer and David Nalbandian in slow motion handling high balls on the backhand side so you can pay attention to their preparation and a different racquet path through the ball.
The usual mistakes when dealing with a high ball to the backhand or forehand are hitting it down – which brings the ball in the perfect zone for your opponent to attack, or hitting it back up which sends a passive ball over the net and allows the opponent to dictate again.
Hitting the ball horizontally enables you to hit with some pace and neutralize potential attacks.
Check out this video — it is almost 12 minutes long.