The good news is this is information can be updated directly in your account! You can update the footer in the edit mode of any draft campaign, or sent campaign you copy. When you're in your email and click anywhere in the footer, you will get a pop-up to make edits. This is where you can fill in the fields to update your physical address and click Insert.
I would also suggest making these changes in your Organization Information and Organization Address as well. Updating this section in the Settings tab of your account will update the default information in any new email you create.