Big Medium always displays the most recent articles first, based on the date you set as the publication date in the Article Info panel of each article's edit page. When you create new articles, the publication date defaults to the current date and so, if you leave the publication dates unchanged, Big Medium always shows the most recently added articles at the top of link lists.
However, you may set the publication date to any date you wish. Adjusting the publication date of the article adjusts where the article appears in the list. Changing an article's publication date to a later date will float it up higher in the sort order. Likewise, changing it to an earlier date will make it sink lower in the sort order.
For example, if you have an old article that you would like to promote again on the homepage or in a more prominent position elsewhere on the site, change the publication date to today's date, and the article will appear at the top of the list of articles along with today's other articles. Note, however, that this will also change the posting date shown to your visitors.
Need more help?
For further assistance and documentation, visit the website of Global Moxie, the developer behind Big Medium.