To change your 4-digit PhonePe passcode,

1. Tap your profile picture on the top right side of your screen. 
2. Tap Manage Your PhonePe Account. 
3. Tap Change Passcode.
4. Enter your PAN and date of birth as per PAN and tap Proceed.
5. Enter and re-enter your new passcode.
6. Tap Save & Proceed. 
7. Enter the OTP sent to your mobile number.

If you have forgotten your passcode, 

1. Tap Forgot Passcode.
2. Enter your PAN and date of birth as per PAN and tap Proceed.
3. Enter and re-enter your new passcode.
4. Tap Save & Proceed. 
5. Enter the OTP sent to your mobile number.