Notes password recovery

According to the Apple Support document you can either lock your Notes with your iPhone Passcode or create a unique password to lock your notes. If you use a unique password, you will not be able to access your notes if you forget this password.

Lock or unlock notes on your iPhone or iPad - Apple Support

The relevant portions are here:

If you use a unique password for Notes, there is no way to access your locked notes if you forget the password. 

Apple doesn't have access to locked notes and can’t help you access them if you forget your notes password. If you forget your notes password, resetting the password doesn’t give you access to previously locked notes. If you reset then create a new notes password, that only allows you to lock other notes.

See this article. You may create a new password affecting new notes, but without that old password you cannot access the existing notes.

Lock or unlock notes on your iPhone or iPad - Apple Support