The Happy School Days Demo displays the Simple Social Icons plugin in the Header Right Widget. Go to Dashboard > Appearance > Widgets > Drag a Simple Social icons widget into the Header Right widget area.
NOTE: there is no need to select any colors for this widget, as the demo has the icons coded to display in multiple colors. If you wish to select your own colors, you will need to remove the coding from your stylesheet, and then update the widget directly with your desired colors.
Add your links to your own social media accounts in the appropriate location.
Be sure to click the Save button to preserve your settings.