If your InstaPay transfer shows as successful but you sent to the wrong person, there's no button within the app to reverse the transaction after it's successful.
Contact your bank immediately and keep the transaction number, time, amount, and the beneficiary's details.
Ask them to explain the possible procedure for trying to recover the funds.
Do not send a second transfer to "correct" it, and don't assume the bank can automatically reverse the transaction; once a transfer is successful, a refund is not guaranteed.
The figures, fees and limits in this answer vary by bank and change over time — confirm them with your bank or the official source before acting.