You can filter stories by topics via a top filter All Topics. If you have previously selected a country for trending stories, the dropdown for topics will only show topics which have stories in this country.
To choose a topic:
Click on the related drop down menu on the top left corner of the page with trending stories and select the topic by which you want to filter and click on it.
On Trending screen tap on the tab of the topic you would like to explore.
On Trending screen tap on the tab of the topic you would like to observe.