Admin MOTD is one of the server administration windows. Open it from File > Admin; it opens as a desktop window you can drag, resize, minimize to the taskbar, and close with its title-bar X.
Setting it
The pane at the top shows the message that is set right now, so you can see what you are about to replace. Set MOTD writes the editor's contents; submitting an empty editor is the same as clearing.
Clearing it
Clear MOTD removes it entirely — users connecting afterwards see no message of the day until one is set again.
Access
Only server administrators may edit. Server operators can read the current message but the editor and both write buttons are disabled.
See Also
admin console · /motd · /setmotd · /clearmotd