Hi Mikako,
To set the video slider first you have to activate the Revolution Slider Plugin where you can purchase the single license from here.
After that activate the plugin in the site and create a slider via revolution slider option and go to Appearance > Customize > Ambition Theme Options > Featured Content Setting and choose Revolution Slider option then go back and refresh the page you will see the revolution slider option where you can set the revolution slider that you have set.
Thank you!
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