How can we challenge the Will in the Court?

Hello Sir, I would like to know about family property contribution and in which cases we can challenge the WILL. because in my family My there are 3 sons and 3 daughters of my grand father. Firstly my Grand father make an Will in which every children will get equally part for his property and assets and my grandfather died in year 1999, after after his death my uncles show a new will which was signed by my grand father in which written that all property will divider to only his two son except my father and his 3 sisters now what we can do in this case and if i like to read similar case study