A video demonstration for this topic can be found in your website dashboard, under Video Tutorials > Yoast SEO Tutorials

You can access this option by clicking on the Social or Share Icon on the left hand side of the Yoast SEO panel. This will allow you to see the preview tabs for Facebook and Twitter.

On each corresponding tab, you can also edit how the post or page will appear on Facebook or Twitter. You can change the image by uploading a different image, edit the title and description.

By default the featured image of the article will be used. If there's no featured image, the first image of the post will be used instead.

To know more of each items' description, you can click on the question mark icon next to the item.