Hi ​@Prabhat Kumar, thanks for reaching out! 

I totally understand how it feels to make a mistake. While we’re not able to assist with billing here on the forum, I took a look at the account associated with email of your forum account, and it looks like you’ve already reached out to support on ticket #1545477.

They replied to you earlier today to verify which team the dev subscription belongs to so they can help cancel. Please check your email inbox at your earliest convenience and reach out to them directly to cancel the seat.