To make your voice sound higher you will want to practice relaxing your throat muscles for easier voice control. Then you can apply vocal and breathing exercises to learn engage the right muscles and increase your vocal capacity.
Finally, utilizing your nasal cavities, practicing reading and recording yourself will help to fine-tune the high voice you want.
Relax throat muscles
First of all, to be able to control your voice better you need to make your upper body muscles become loose and flexible. Try relaxing your whole upper body, mouth and throat muscles with stretching exercises and yawning.
The relaxed muscles around your larynx will allow more flexibility and will expand your vocal capabilities, making it easier to achieve a high voice.
Engage and release throat
Perform this exercise whereas you create “ahh” sound by engaging vocal cords and then you release your vocal cords for a released sound.
This exercise will teach you to control your vocal cords better. I’ve added a video for more details.
Once you master the technique practice the exercises in varying pitch by going as high as you can.
Breathing exercises are great to increase your lung capacity. By default, we usually breathe shallow breaths.
Try implementing diaphragm breathing to involve more muscles and provide more air into your lungs. This will help to maintain a high pitched voice for longer.
Otherwise, you risk lacking air to maintain the change in pitch which is more demanding than your natural voice.
To give your lungs more air try strengthening your diaphragm by laying down on a bed and putting some weight onto the belly, then breathe using the diaphragm muscles.
You will know that you are breathing through the diaphragm once it feels as if your tummy is filling up when breathing and your shoulders don’t rise as much.
This exercise will give you more air to work with and set you up to maintain a high-pitched voice for longer.
Use your nasal cavities
Try speaking more through your noise which will add some nasal qualities to the voice.
Nasal voice sounds more brittle and higher-pitched. When speaking you should emphasize resonating within the upper part of your face mask where the nose is.
Practice reading in a higher voice
Practice using a high voice when reading. Pick up any book or article and read it in your highest voice. Continue reading with a high pitched voice until your voice gets tired.
Spending 10 or so minutes daily reading in a high pitch voice will make you feel more comfortable using the voice and it will start sounding more natural once you get a hang of it.
Record yourself to see how your voice develops and track your progress while achieving a high voice.
In addition, there are apps such as Vocular that can measure the pitch in which you are speaking. This will help to accurately assess your progress and motivate you.
When recording you will notice flaws that you make, which otherwise can be unnoticeable.
If you don’t perform live you can use EQ after recording to make your voice sound higher.
Try cutting all frequencies below 60Hz as they are redundant anyway. Then reduce all frequencies up to 500Hz by 3-4dB. This will reduce low-end frequencies and will make high-end frequencies of your voice more audible.