Can I Get a Will Changed for a Person Who Has Alzheimer's or Dementia?

No. You have to be legally competent to execute a will.

Even someone who has guardianship cannot create a will.

However, an individual can create a trust that can substitute for a will. To do so, someone will have to be appointed and then seek court approval for the new estate plan. You might also be able to challenge the new will if you believe the person was not competent or under undue influence at the time she executed the new will.