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.
If not, and if he's competent, get a new POA made immediately. It'll make life easier if he's well enough to do that. If he's healthy enough to go with you to his bank to fill out their POA paperwork, even better, because banks can be real sticklers about accepting perfectly legal Powers of Attorney that aren't theirs.
A living will isn't the best thing either. An Advance Medical Directive is better.