Iowa Code § 633.284

A will can be revoked in whole or in part only by being canceled or destroyed by the act or direction of the testator, with the intention of revoking it, or by the execution of a subsequent will. When done by cancellation, the revocation must be witnessed in the same manner as the making of a new will. No will, nor any part thereof, which shall be in any manner revoked, or which shall be or become invalid, can be revived otherwise than by a re-execution thereof, or by the execution of another will or codicil in which the revoked or invalid will, or part thereof, is incorporated by reference.