Increasing the frequency, or pitch, of your whistle requires that you increase the tension of your lips when whistling. Less tension will create a lower pitched whistle. while more tension will create a higher pitched whistle. To whistle effectively in the high range, you must have a great deal of breathe support and control over your embouchure. An embouchure consists of the muscles in your face that are used when creating a whistle.
Position the tip of your tongue so that it almost touches the roof of your mouth. The tip of your tongue needs to bend slightly downwards and the flat part of the tongue must come close to touching the roof of your mouth. The tip of your tongue should line up with your bottom teeth.
Make a "woo" sound with your mouth and hold that position. You should notice that your lips create a circular hole in the center. This shape is crucial for creating a whistle.
Blow air through your lips until you can create a successful whistle.
Increase the pitch of the whistle to create a high-frequency whistle by bringing the tongue further back into your mouth and increasing the tension of the lips.
Make sure your lips are moistened when whistling.