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, Button Text and the Icon Class. The Description and icon will display to the left with the optional button to the right.  If the link/URL is used the button text is required. 

A list of custom icons can be found 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.