Transfer of property after death of grandparents to father's name

The flat where I reside is owned 50% by my father, 25% by my grandmother and 25% by my grandfather. My grandmother and grandfather expired last year. They had made a will in favor of my father to transfer the entire share in the flat in my father's name. But this will was not probated.. My father has 1 brother, 2 sisters and a widow and 2 children of my father's another brother.. Should we go for probating of will? Or should we go for relinquishment deed? What is the procedure for each? What would be the approximate expense for each? What would be the cost and procedure of transferring the share in my father's name after having done one of the above two procedures?