Hi @PeterGrahame! The way that I have changed it before is by logging into the Square Dashboard first. From there, you want to click on your business name in the upper right corner and click on Account Settings.
Inside Account Settings, click on the Personal Information off to the left side of the screen. The first item should be Business Name and option to select the Edit button. Doing this will allow you to change your DBA name inside of Square.
Hope this helps!
