From Online Manual
If enabled, the forum news will show in the header of the forum. The news will display the content of the text boxes found on the news management page (Administration Center > News and Newsletters > News). By default, it says “SMF - Just Installed!”.
To edit the news, the admin simply needs to edit the text inside the text boxes on the news page. Both Bulletin Board Code (BBC) and HTML can be used to format the news in these boxes. To add more news items, the admin can select the Click here to add another item link located at the bottom of the news page. To delete news items, the admin can select the check boxes beside any news items, and select Remove Selected.