Each announcement is posted at certain date & time, thus this piece of information is part (meta-data) of that announcement, news, or post. Thus the presentation of announcements are ordered by the posted (created) date/time: newer one at top. Think of it like daily newspapers, which are published every day, once published and available at newsstands, it's impossible to re-arrange the publishing dates. Similarly, it's not in the best practice to "re-arrange" the order of announcements being posted, since announcement is a piece of news, of which posting time/date is an essential part. Most common practice is to create another announcement to re-state an important news, or a prominent link to that old announcement.
With that in mind, and if you still want to re-arrange the order in which announcements appeared, best way is to: