If a trust provides a procedure or method for amending the trust, whether phrased as exclusive or optional, any amendment must conform to that procedure or method; otherwise it is invalid. Prob. Code 15402’s “qualification ‘[u]nless the trust instrument provides otherwise’ indicates that if any modification method is specified in the trust, that method must be used to amend the trust.” Here, the trust required notarized signatures for any amendment. An amendment with unnotarized signatures was void. The opinion disagrees with Haggerty v. Thornton (2021) 68 Cal.App.5th 1003, review granted, on this point.