Refund not given to me

I had a prepaid SIM card connection. Without my permission or consent, my prepaid connection was converted to a postpaid connection. I did not request or authorize this change at any point. This conversion was done by Airtel without my knowledge, and I only came to know about it later. I demand that my connection be restored back to prepaid as it was originally, and I request that Airtel take action to prevent such unauthorized changes from happening again. I also request an explanation from Airtel as to how this conversion was done without my consent.