A family of Microsoft word processing software products for creating web, email, and print documents.
Right-click the merge field.
Select Toggle Field Codes from the popup menu.
You'll see something like { MERGEFIELD "DOB" \* MERGEFORMAT }
Change the latter part so that it looks like { MERGEFIELD "DOB" @ "dd/MM/yyyy" }
Press F9 to hide the field code and to update the result.