Airtel — Delay in plan changes and generating incorrect bills

I had requested for the regular six monthly plan change three months back. But airtel kept on generating monthly bills, after calling numerous times to customer care, they kept informing plan has been changed. I am paying unnecessary taxes on each bill generated. They raise complaints and close by themselves without calling and checking with customer.
Recently the raised disconnwction request on account by themselves while the payment of 6000 has already been done.
REQUEST FURTHER INVESTIGATION ON THIS PLEASE.