You can edit the Alert bar by going to Content > Link Lists > Alert Bar and adding a new item.
You will need to add the Name, Link, Description and Icon Class. The Name will display as the link text, and the Description displays before the link. The Icon will display at the beginning of the alert.
A full list of custom icons is available here:
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.