A will is a person's last chance to tell you what he or she wants to do with his or her assets and estate. Normally, it's extremely hard to challenge a will, because attorneys take the time to have witnesses present and to guarantee a person's ability to sign a legal document before allowing one to be completed.
In the case that you believe your loved one's will was changed or altered fraudulently, you do have a right to challenge the will. For instance, if your loved one was suffering from dementia and another party lured him or her into making changes to his or her will, you could produce documents to show that your loved one couldn't have made that decision without provocation.
Challenging a will is a good idea if you believe that your loved one was manipulated or coerced into signing it. It can even be a good idea if there are multiple wills and you aren't sure which one is legally valid. Your attorney can help you by helping you gather supporting documents for your claim.
For example, you may want to have copies of older versions of the will, medical reports that describe your parent's medical condition and mental state or other documents to show that he or she signed a document that does not represent what he or she would have wanted to do for his or her beneficiaries. With good documentation and a strong story behind your challenge, there's a better chance that the court will hear the issue and allow the will to be amended to reflect your loved one's true wishes.
Source: FindLaw, "Revoking, Challenging or Changing a Will," accessed Nov. 24, 2017