You can change the SIM type from prepaid to postpaid by following these steps below-
- Tap the SIM you want to change on Home screen. You will be directed to your SIM details.
- Under the name of your operator and circle, you will see 3 Tabs. Tap on the last one, 'SIM Settings'.
- On the SIM settings page, you'll see your SIM type set as Postpaid. Tap it, and change it back to Prepaid.