How to set order of posts by date

Having an issue with this theme, Atahualpa, and having it display posts in descending order by date….that is, newest first to oldest.
Have tried a few plugins, where you can drag the order around, but they do not work.
Anyone have a resolution, such as editing one of the them files, to get the posts to display the way mentioned above?
Atahualpa’s default sort seems to be oldest to newest.

Many thanks.