To modify the theme, you have to open the customize page. If you don't know how to do that, please refer to here.
To create an about page, just go to Online Store > Pages. When you are editing the page, just choose the template is one of "page.about1", "page.about2", "page.about3".
Each about page template display the correspoding "about" section. To edit "about" section, just go to Online Store > Customize. Choose about page you created. Then click "About" section on the left and edit it.