Airtel — postpaid connection plan changed without my consent

I had opted for Airtel postpaid plan of Rs 199. This was changed to 399 without informing me or without my consent. I never asked for this change. I just realized that I have been paying this extra unnecessary money for the last 8 months or so. I want a complete refund of this extra amount along with a letter of explanation and apology. This isn't the first time this has happened with Airtel.