Can my parents override their will on their own with a new letter?
They can override thier own will with a new will, but not with a letter, even a notarized one. The new will doesn't need to be long or complex, but it does need to be executed (signed) and, very importantly, properly witnessed as well; as being notarized. A will can be changed or overridden only by a new document meeting the criteria to be a will.