1. To be aware of high tone vocalization, and make repeated practice.
General way is, if you inhale enough air to the esophagus and put a pressure on your abdomen to push the air toward the upper part of the throat to let it out through the nose and vocalize. This makes, without doubt, surprisingly louder voice.
It is effective if the practice is done daily, and in addition, if you, for example shout to the family members upstairs several times, or to call people in a distance, or dare to carry on conversation in noisy train, etc. Taking every chance of practice is a good means to attain high tone. In short, you make good use of opportunities in daily life which you can make high tone of voice.
2. To identify the exact region of vocalization in the upper throat, where you can feel the slight vibration by putting your fingers while vocalizing.