Nintendo Switch Manual page 49

You'll receive an enquiry number, which you must
enter in the Customer Support to be sent a master
key which is used to reset your PIN. Select "Enter
Master Key" to input the key and choose a new
PIN."
Caution when resetting your PIN
If you enter a master key to reset your PIN while
the Nintendo Switch Parental Controls app is
linked to the console, the app will be unlinked,
and play activity and play time restriction settings
will be deleted.
Console settings for gameplay restrictions will
continue to exist.
Please have a parent or guardian call Customer
Support.