Hello There is  no natural method to reduce eye power ( myopia) As you are already 21,power won't increase much now. Refractive surgery like lasik is an option However, try to reduce screen time and increase sunlight exposure. That might help in stabilising myopia

It may not increase in view of your age.There is no method to reduce.you can have lasik surgery. Pl consult ophthalmologist for further evaluation and treatment.

Minimize screen time. While using digital devices, every 20 minutes, take a 20 second break and look at a distant object 20 feet away. Maintain right posture while using computer. Keep yourself hydrated. Eat healthy nutritious diet with lots of fruits and vegetables. Blink more often. Sit away from ac and fan, to prevent dryness of eye. To consult sos.