If you have included a date field in your letter such as Appointment Start Date or Today's Date, you can update the format simply by double clicking on the reference.
You will be presented with the following window:
Under the OutgoingLetter option, scroll down until you find LetterDate and click on this to view the following options in the Format section:
You can then simply select the new format you wish to use and select OK. Your date format will be updated in your letter for this reference.