To modify the theme, you have to open the customize page. If you don't know how to do that, please refer to here.
On the customize page, click "Theme Settings", then click "Header".
At there, you can upload a logo for desktop and mobile screen, change the header style and mobile navigation, turn on/off the sticky menu.
- Enable the sticky menu : enable the sticky menu when customers scroll down on the site.
- Wide display : display the header in the wide layout.