You can edit the Alert bar by going to Content > Link Lists > Alerts and adding a new item.

You will need to add the Name, Link, Description, Button Text and Icon Class. The Button Text is required if a link is added and will display after the description text.  The Icon will display at the beginning of each alert.

To turn off the alert, simply remove the link from the link list. DO NOT DELETE THE ENTIRE LINK LIST. You only need to remove the link. To turn it back on, you can add a new item to the link list.