From the dashboard, select Manage Courses, and then View Sections for the appropriate book and course.
Here you can find My Sections and the link to Announcements.
The Active tab shows active announcements, while the Create New Announcement allows posting to a single, or multiple sections.
If you wish to remove an announcement, go to Announcements. Click on the X button in the upper right-hand corner. You will be provided a warning screen before the announcement is deleted.