How can I unlock a note, where the password of that note was not saved earlier?

You can't. From the Related Article above.

To unlock a note, you need to enter the password that it's locked with. 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. A new password will let you password protect any notes that you create going forward with the new password.