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, the Link, the Description, and the 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 list of custom icons can be found in your Setup Guide here.
To turn off the alert, simple 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, simply add a new item to the link list.