1. Login to airtel selfcare from a Desktop or Laptop. Don't use Mobile or Tablet.
2. After logging in under connections, see if your TopUp appears in HD- Topups or SD-Topups.
3. If you are able to see your TopUp, click on the price mentioned (Eg: Rs 99/mo). It will show a pop-up with option to cancel the Topup. No refund will be issued for remaining days of the Topup.
4. In some cases, like Promotional topups which were activated through MyAirtel App, it doesn't show in the Desktop selfcare at all (I hate airtel for this). In such cases I don't think there is any way to deactivate using Selfcare. You need to call CC.
Hope this helps.
Cheers.