What should i do to increase my height ? i am 21yrs old now.

Depends: If you are in your teenage years or if you are referring to a child, a good healthy diet with plenty of exercise and activity can maximize your chances to attain your full potential. However, if you are an adult, there isnt much you can do as your bones have fused leaving no room for further growth.

Unlikely to happen: At 21 the growth centers have likely closed so no more linear growth is expected.If you want to know for sure an x-ray of the wrist ; hand can be analysed for bone maturity ; tell if the growth centers remain open to new growth. A bone maturity more consistent with 15 would leave some room for growth.