If you've forgotten your 4-digit PIN and/or want to change it, just follow these simple steps:
You can view or change your PIN on your Profile page, which can be accessed by opening the menu in the Noonlight app (top left corner on the home page) and tapping on "Profile"
Once on your Profile page, tap on the PIN field, which is located underneath your phone number
Enter a new PIN in place of your old/current PIN if you need to change it
Be sure to tap Save when you are done