I'm going to assume that you want users to be able to type into the Region field (i.e., that options other than "San Antonio" should be available). If that's right, then you can set up an auto-enter calculation in your Manage Database dialog (click on the Region field and choose "Options"). The calculation should look something like this:
Case ( Address City = "San Antonio" ; "San Antonio" )
That way, if someone selects "San Antonio" for the City, it'll automatically insert "San Antonio" in the Region.
If you want to update the existing data, perform a Find for all records where City = "San Antonio". Then use Replace Field Contents (Ctrl / Cmd - =). Set the calculation for the Replace to be:
That will cause FileMaker to replace the contents of the Address City field with its current contents. (Huh? What good is that?) Well, it'll cause the auto-enter calculation to trigger, meaning all your "San Antonio" entries in Region will be updated without risking doing any damage to existing data. (Just make sure the cursor is in the Address City field before you hit the Replace.)
Thank you so much for your help. Saved me HOURS!