Not simply anybody may challenge the validity of a Will. State laws differ a little, all require that you be related to the Will in some manner. Generally, you must be able to reveal that you would have received more from the decedent, if she or he had actually passed away without a Will; or if she or he wrote an earlier Will that left you more than the present Will.
Question 2: When can I challenge the Will?
Question 3: How can I overturn a Will?