This theme requires you use a logo or image in lieu of a text header.
The required dimensions are 800px wide by 400px tall.
Navigate to Appearance > Customize > Header Image.
Select your image, upload, save.
Your logo will not appear until you create your navigation menu, and place a home link in the center.
When you create your "home" link in the primary navigation tutorial, be sure and use https if your site has an ssl certificate installed, otherwise your logo will not show up. If your logo is not showing, try changing your custom link from http to https, or vice versa.
An editable photoshop file of the demo logo comes with your theme (yay!), you'll find it in your theme zip under "PSD FILES", you can open that in photoshop and use the text tool to add your own text in whatever font you like. The font used in the theme is called Flower Child and is available for purchase here: Get Flower Child Font.
If you'd like to use the image from the demo, you may right click on it below and save it to your computer for uploading.