Forgot password of memo

Are you asking about the Notes app?

About setting the password - Apple Support:

"Important: If you forget your password, the only way to view a locked note is by using Touch ID, an approved Apple Watch, or Face ID (if you have a device that supports one of those features and you enabled it before you forgot your password). For that reason, it’s strongly recommended that you add a hint when you create your password. Notes shows the hint after three failed attempts."

Lock your notes on Mac - Apple Support explains how to Reset your password.