Open the pasby-app and follow this steps:
Select "Settings" in the bottom navigation bar.
Underneath active pass select "Change security code".
Choose a new, 6-16 digit pasby code that only you know.
Did you forget your pasby code?
Then you need to block/deactivate your pasby for that device and then activate a new one. You can also do this by deleting your pasby-app, reinstalling it via the Apple app store or Google Play store and activating a new pasby.