How to get a POA for my Mom (86)?

My mom has a case manager and he is acting like he is in control. He decideds for her in almost evertyhing with out consulting me. ex medical needs and housing needs. My mother is complaining because he can not eat what she wants whe she lives and want to live with me. I am not sure if mom already signed a POA for this case manager. My mon is 86, but can still carry a conversation. even sometimes , you ahve to explain to her situation several times. I want to ahve a POA , so that I will be the one oversee what is best for my mom.