Select the Announcements tool from the Tool Menu of your site.
There are five options that allow you to auto-sort the Announcements:
When you click the link, an arrow icon appears beside it showing if the list is sorted smallest to largest, or largest to smallest. In the above illustration, the announcements are sorted by modified date, with the oldest at the top and the most recent at the bottom. If the link is clicked again, the arrow icon will point down showing that the most recent announcements are at the top and the oldest ones are at the bottom of the list.