When you edit an existing Notice (click on notice title or Edit under Actions), you'll be navigated to the Editing screen.
You have 3 main edit functions with buttons in the middle of the screen: Edit, Preview, and Tag.
Under Edit, all the steps used to create the Notice are available in an accordion view. Simply click the grey arrow to open the section you desire to edit. Note: These provide the same actions/steps used to Create a New Site Notice.
Under Preview, you can see exactly how the notice will look on the webpage.