The theme shows default / sample widgets in the “Sidebar – Default” position. In order to remove the sample widgets, you can assign your own widget to the “Sidebar – Default” position. If you do not want to assign a widget to this location, then alternatively you can turn off the “Example Content” under Appearance > Customize > General Settings.
Please note – when you turn off Example Content, all sample/default images that show up for posts that do not have featured images will also stop being displayed. So please make sure you upload a featured image for all your posts. More information on how to upload featured images: https://en.support.wordpress.com/featured-images/