Airtel Postpaid — change postpaid plan

For the past few months i am trying to change my airtel postpaid plan. I want 499 per month plan. Currently, i am having 999 per month plan. My uasge is very less and so i am paying unwantedly high amounts every month. Also, the network speed is very pathetic inspite of having 3g/4g connection. Kindly do the needful asap. My airtel no is [protected]