Airtel — receive sms for upgrade monthly plan without consent

Address:

Thane, Maharashtra, 421301

Today i received below sms from airtel

"dear customer, starting 25-nov-18, your airtel mobile 977381* will be upgraded to an unlimited calling plan with 5gb data at just rs.299/month. Not only this, you get 100 sms/day and rs.50 discount for 12 months. To check other plans, click www.airtel.in/infinity"

My question is how can you change my monthly plan without my permission?

I don't want to upgrade my monthly plan. Kindly take necessary action on urgent basis

Prajyot mane