If you have a date field in your letter (e.g. appointment date, letter date) you can change the format simply by double-clicking on the reference within the letter.
On the left, scroll down until you find the appropriate field you are wanting to change. This should already be highlighted and simply needs to be clicked again.
Once you click the field to select it, you can choose the desired format on the right as shown below.
Click OK and the date format for this reference will be updated in your letter.