The term "Upper Back" refers to the region of your spine half way up your trunk to the base of your neck. Unfortunately it is all too common to see poor posture in this part of the body.
One typical poor posture for this area of the body is rounding of the shoulders. This basically puts the muscles of the upper back in a lengthened position that eventually causes them to loose strength.
There are many larger and smaller muscles of the upper back but the main muscle we want to focus on in strengthening is the Trapezius. It is one muscle but can be divided into three parts: Upper, Middle and Lower.
It is most common for the Middle and Lower portions of this muscle to become weak.
The video shows a way to exercise these parts of the muscle.
The key to these exercise is not to lift the arms up from the shoulder joint but to keep the arms heavy and create the movement by squeezing the shoulder blades together.
Do 10 times, build up to 3 sets of 10.