If you forget your PIN, it's possible to reset it by following these steps:

1. Open the Parental Controls software from the Wii U Menu.
2. Select Parental Controls and tap "Forgot PIN".
3. You will be prompted to enter the answer to your secret question. Once entered, you will gain access to the Parental Controls front page. You can reset your PIN here by tapping "Change PIN" and following the on-screen instructions.

If you have also forgotten the answer to your secret question, you can have a master key sent to your registered email address.

Nintendo’s Parental Controls PIN reset tool will generate a master key to reset the Parental Controls PIN on your Nintendo console or system.

If you need a master key, please go here.