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).
The "description" displays to the left with the optional url/button to the right. If you use the "link" field, then the "Button Text" field is required. The entire bar will point to the link url.
The icon class is optional and will add an icon at the beginning of the alert.
A full list of custom icons is available here: https://mk047.monkpreview.com/_assets/fonts/icons/icomoon/
To turn off the alert, simply change the Alert Bar title (ie: Alert Bar - OFF) or 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 change the title back to "Alert Bar" or add a new item to the link list.