Under the Postpaid tab on Airtel website, select Switch Prepaid to Postpaid. Choose the Postpaid plan and click on Buy. Enter your registered Airtel number and other details. After successfully submitting your KYC documents, your new postpaid connection will be activated within 48 hours. Similarly, you can repeat the process via the Airtel Thanks app, wherein you need to choose the Prepaid to Postpaid option under the Postpaid tab and enter your registered mobile number and other details. You will get an OTP and you must select a postpaid plan as per your choice. Your plan will be activated in 2-3 working days.