my mother has mild dementia and wants to change her will. Can she do it?

You would have to document your mother's mental condition.  A mild case of dementia is not necessarily going to prevent her from redoing her will.  She must know the "objects of her affection and the nature of her bounty" which means she has to know what assets she has and to whom she wants to leave them.  Even if she has some confusion or other issues, she might be lucid enough to do a will.  That would be a very fact specific decision.