If you want to preview an announcement, you can do this by creating the announcement without publishing it. It will be saved as a draft in the 'Announcements' tab, and here you can enter the announcement to preview it - you preview it by pressing on the title:
To make changes and/or to publish it you press the pen icon:
Make the changes you wish to make, and if you are ready to publish the announcement press the 'Publish' button.