How can someone cancel power of atty and get it in their name when dementia is involved.

If you have a valid power of attorney,  only the person who nominated you for this purpose is legally allowed to revoke it. If your son has brain damage and is not legally competent, he would not be able to revoke your current power of attorney papers  and or have been competent to have done a new power of attorney to appoint his daughter. It sounds like his daughter has arguably taken advantage of the situation and may have wrongfully taken his money and assets. You should carefully review your current power of attorney papers to see if you have  rights under the document to conduct legal matters for your son, including the commencement of a lawsuit in your son's behalf. I would suggest that you consult as soon as possible with an experienced estate lawyer to determine what rights you may have against your grandchild, who presumably is an adult.