How do I edit the ssh motd?

The files you need to edit live in /etc/update-motd.d.

See the update-motd man page for details about how it works.

My version of Ubuntu doesn't show the disk usage, so I'm not sure which script exactly you need to edit.

Take a look at the Ubuntu Wiki article about the design of the update-motd application. This program is contained in the update-motd package and will help you to create a more dynamic MOTD.

You can't edit the Script without extra coding. The message you see is shown with a wrapper script which calls /usr/bin/landscape-sysinfo. The output you see is created by landscape-sysinfo