Unlocking old notes

You can't, sorry.

See this paragraph:

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 won't give you access to previously locked notes. If you reset and then create a new Notes password, that will only allow you to lock other notes.

Extracted from this support article:--> How to lock or unlock notes on your iPhone or iPad

Regards

Giulio

You will need to remember the original password or you won't be able to unlock the note.

You can change you Note password, but If you forgot the password, Apple can't help you regain access to your locked notes. You can create a new password, but this won’t give you access to your old notes.