Open the Google Pay app
Tap your profile picture in the top right
Tap Bank account
Select the bank account linked to your UPI
Tap Forgot UPI PIN or Change UPI PIN
Enter your debit card details if prompted
Enter the last 6 digits of your debit card
Enter the card expiry date
Verify with the OTP sent by your bank
Set your new UPI PIN
Re-enter the new UPI PIN to confirm