How to reset forgotten Notes password on iPhone

See the following from Keep your notes secure with password-protection - Apple Support

If you forgot your notes password

You can unlock a note only when you enter the password that it's locked with. If you forgot your password, Apple can't help you regain access to your locked notes.

You can reset your password, but this won’t give you access to your old notes. It will, however, let you password protect any notes that you create from that point forward with the new password. This could lead to you having multiple notes with different passwords.

If you have multiple passwords, you can't tell which password you should use when you look at your notes in the Notes list. When you open a note locked by your old password but enter your current password, you'll see an alert that you entered the wrong password with a hint for your old one. If you then enter the correct old password, you get the option to update that note's password to your current one.

Reset your notes password

When you reset your Notes password, it doesn't delete or change the password of your notes that are already locked. If you have a note that uses an old password, you can update it to the new password when you enter the old password, remove the old password, then apply your new password to the note.

On your iPhone, iPad, or iPod touch:

1. Go to Settings > Notes > Password.
2. Tap Reset Notes Password.
3. Enter your Apple ID password.
4. Tap Reset Notes Password again.
5. Enter your new password and password hint.

On your Mac:

1. Open Notes.
2. In the menu bar, click Notes > Reset Password > OK.
3. Enter your Apple ID password.
4. Click Reset Password.
5. Enter your new password and password hint.