To edit existing announcements, go to the announcement manager at Announcements > Announcement Manager.
This page is divided up into two sections. At the top you will see Global Announcements, if you have any. These announcements will be displayed at the top of every forum. Click to add a global announcement.
Below this, you will see a list of each forum. If you have any Forum-Specific Announcements, they will be displayed next to the appropriate forum. These announcements will be displayed in this forum and its child forums. Click to create an announcement for just this forum.
The information displayed with each announcement is its title, the user who posted it, edit/delete controls, and the timeframe that it will be displayed for.