I don't know the password of my deceased brother's iPhone

Hey I have my brothers iphone. I don’t know his phone password or Apple ID. Would like to erase the phone so that I could use because I believe he paid for the phone at Walmart and was just buying service when needed. He was unable to give me this information because by the time I got his phone he was not capable of giving information. He has now passed and I have his death certificate as well as being his executor of his will and power of attorney. Can someone help me?

[Re-Titled by Moderator]