The easy way is to change xhtml setup for the tt_news extension is to back up the div-based tt_news template, and put it somewhere in the fileadmin folder, where you have placed other template stuff such as your general xhtml template for the website. Make your markup changes to the template, and change the tt_news configuration to use the modified template like below.
### Template File ### plugin.tt_news.templateFile = fileadmin/templates/ext/tt_news_v2_template.html