Update (Custom) Date Field in Word with VBA

With the following macro I can add a Date Field with a date on a workday in the future (how far into the future is realised via a input box):

Dim InputDays As Single: InputDays = InputBox("Due in x Days", "Due Date", 14)
Dim DateDif As Integer: DateDif = Fix(InputDays)
Dim temp As Integer: temp = 0
Select Case Weekday(Date + DateDif)
    Case vbSaturday
        temp = 2
    Case vbSunday
        temp = 1
End Select
Selection.InsertDateTime Format(Date + DateDif + temp, "dd.MM.yyyy")

But if there already is a Date Field at the current cursor position, how can I update the existing date with a new one?