Select a message from the dropdown list.
It can be an active or inactive message, it does not matter.
Once the message has been selected, it will automatically fill the subject,
message, attachment, SLURL, message settings and the IM message

It will also select all the groups you checked before you saved your message.

Modify any part of the message or change groups according to your needs.
Then click the modify button.

The timersettings window will appear again.

Make any changes in here if you need to and then click save.