Union has no problem during editing the column.In the Update Property Dialog Box,you need to specify which table you want to update. Let say if there are two table in the UNION and if you see only one table in the update property table list, just enter the table name in the table to update list box and save it.Now make the column as editable by changing the tab order from 0 to any positive number resulting making that column editable.
Now when you will point the cursor over that column, it will let you change the value. Change with new value and update. It will update your new value in the table.