You can edit or update fields at any time by proceeding as follows:
Editing fields
If you want to edit an existing field, for example, to change its date format, simply select the field in the text and choose the ribbon command Insert | Field again. Alternatively: Choose References | Field or Mailings | Field.
Again, select either the path via the small black arrow of the dropdown menu icon or click directly on the icon itself to open the dialog box. Make the desired changes, and the selected field in the text will be replaced accordingly.
Tip: You can also open the dialog box by double-clicking directly on the field.
Updating fields
When you open and print a document, all the fields it contains will be updated automatically. Thus, if you print a document containing the "Print date" field tomorrow, tomorrow's date will appear in the field.
You can manually update the fields at any time by choosing the ribbon command References | Update fields 
or by pressing the F9 key. All fields and calculations in the document will then be updated.