The News & Information section of the Welcome screen allows system administrators to broadcast information to users, but new messages tend to be displayed at the bottom of the list. The display order is driven by the Start date. Active messages are displayed in chronological order with the earliest or oldest start date first. So if you want your new message to be at the top of the list, make sure it has the oldest start date!
Messages are created and edited on the Broadcast Message Management screen which can be accessed through the Message Mgmt navigation link located under the ADMIN icon.