First, be certain the team you are in allows you to delete a message. Just because you are given the ‘Delete’ option does not necessarily mean your team is set to allow deletions. For information on how to change this setting, see TEAM OPTIONS.
You can delete a message by selecting the message (right click or two finger click on desktop) and choosing the ‘Delete’ option.
The same process can be applied for file attachments. Select the message (right click or two finger click on desktop) and choose the ‘Delete’ option.