Dad has cancer and his DPOA is my stepmom but she is deceased. What do I do to change it so that if we have to make a decision, I can?

My dad was recently diagnosed with stage 4 lung cancer. I am the only child in the state so I have always been the "caregiver", through his back and hip surgeries, etc. He has always said that I am the one to make decisions but of course chat with my siblings first. Well, now he has cancer, and I looked at his paper work. He has a living will, my brother is the executive trustee? but my stepmom is listed as the durable power of attorney, as well as patient advocate. The paperwork was dated 1997. However my stepmom passed in 2015 and he never updated the paperwork. I am his contact on all his medical paperwork, like emergency contact and that but what do I need to do to change it so that if we have to make a decision, I can? He keeps saying I am the one to make the decisions. He does have a DNR, I am aware of that.