A last will and testament, or any clause thereof, shall not be altered, or revoked, except by canceling by the testator, or by some person in the testator's presence and by the testator's express direction, or by a valid last will and testament, or by a writing signed by the testator, or by some person subscribing the testator's name in the testator's presence and by the testator's express direction, and attested and subscribed in the testator's presence by 2 or more credible witnesses; but this clause shall not preclude nor extend to an implied revocation.