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
It is narrated from al-Hasan (may Allah have mercy on him) that when a man makes a bequest, he may change his bequest whenever he wishes. Al-Hasan (may Allah have mercy on him) was asked: What about a freed slave? Al-Hasan (may Allah have mercy on him) said: Whether a slave or anything else.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12655