Once you’ve launched the app, click on “Users” (top right on tab), then click on “Add User”. Type your name as you want it to appear. Fill out the rest of the form. Further down you have the option of setting your status as Author, Contributor, Subscriber, etc.
You can also now go back to any previous posts and in the drop down menu where it shows “admin”, you can now toggle to select your name, which will then update your original posts ??