Attributes can be edited directly from within the Tracker, in much the same way as you might change a filename.
Try this: open up a Tracker window to /boot/home/people and make sure you’re in List view. Pull down Attributes | Person and select City. A list of the cities in which your contacts live should now be visible. Now just click in one of the cities and edit it normally. The next time you open up that Person file the city field will have been updated.
You can do this with any text-based attribute on any file — so long as the Attributes are marked as editable in the FileTypes Preferences panel. For more on that, see the tip: Custom Control Over Attributes.