Step 1: Navigate to Appearance from your admin dashboard then click Customize. Next, hit Header. You can see a list of available header styles.
Step 2: Click on the header style you want to customize.
Step 3: Set the configurations listed on the left sidebar including: Info, Color, Icon, Background, Font settings...
Note: If you have imported the demo, you will not be able to see the changes you made because the page already had a fixed header style.
Step 4: After customizing, do not forget to click Publish button to save your changes.