Most letters default to date created, unless there is a specific field that asks for a date. This is fine if you are sending out the letter immediately, but what if you have prepared it a week in advance and want to update to the current date?
What you need to do is refresh the document which refreshes the created date.
The way to do that is to go back to the work item where the letter was created from, and click the edit icon there (pencil in yellow above), and then save (see below). That will update the date on the document to the current day.
We will perhaps look at more direct ways to change dates in future but for now hopefully this will give enough flexibility.
Of course you can over-ride the field altogether by switching the document into edit mode, delete the date variable and type in hard-coded text.