I have heard back from FTM Tech Support and according to them
the issue does not reside in my source citation, instead the issue is that I did not enter a date in the source itself. As such, the only way to edit my sources in Family Tree Maker is to edit the source itself in Family Tree Maker or Ancestry.com.
In other words, every time a newspaper is cited as a source, a separate source must be created for each separate edition. Apparently the date field on the citation screen has no relevance. I suggested that perhaps it might be better to put the date field to some useful purpose