The Yoast window: > Social > Twitter
just shows the below so cant see anyway to edit it and i did add a default description originally which, if i remember correct just shows when not displaying/tweeting a page etc (which does use the info on the page then)
Note that for the Twitter Cards to work, you have to check the box below and then validate your Twitter Cards through the Twitter Card Validator.
<label for="twitter">Add Twitter card meta data</label>
Add Twitter card meta data to your site's section.
<label class="textinput" for="twitter_site">Site Twitter Username: Client account name</label>
<label class="select" for="twitter_card_type">The default card type to use:</label> <select class="select" name="wpseo_social[twitter_card_type]"><option value="summary">Summary</option> <option selected="selected" value="summary_large_image">Summary with large image</option></select>