Open the menu (≡) at the top left of the Notes screen → "Settings" and turn "Passcode" off and then on again. The "Passcode Lock Settings" screen opens, where you can set a new 4-digit passcode.
Open the menu (≡) at the top left of the Notes screen → "Settings" and turn "Passcode" off and then on again. The "Passcode Lock Settings" screen opens, where you can set a new 4-digit passcode.