You can edit the Alert bar by going to Content > Link Lists > Alert Bar and adding a new item (one item in the list only).
You will need to add the Name, Link, Description. The Name is for internal use only, and the Description is used for the alert content. The entire bar will point to the link url.
The button text and icon class are optional. The button text will display after the description as the link text, and the icon will display at the beginning of the alert.
A full list of custom icons is available here: https://mk044.monkpreview.com/_assets/fonts/icons/icomoon/
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.