Sayyidah Aishah (may Allah be pleased with her) used to say that a man should write in his will: "If the event of my death befalls me before I change this will of mine."
(b) It is reported from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that he said: A man may change his will whenever he wishes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12654