The contents of /etc/motd are displayed by login after a successful login but just before it executes the login shell. The abbreviation “motd” stands for “message of the day”, and this file has been traditionally used for exactly that (it requires much less disk space than mail to all users).
If you want to change this message in ubuntu server follow this procedure first you need to edit /etc/motd.tail file:
sudo vi /etc/motd.tail
Enter your message save and exit the file.