Macro to convert dates format from yyyy mm dd to yyyy-mm-dd

Hello,

I have a 20,000 line Excel sheet...
And all the dates are entered like this:
2009 02 08

I want to add a dash, so they look like this
2009-02-08


so I added a command button, and this code, but its not searching the sheet and adding the dash's...Can anyone help?

Private Sub CommandButton1_Click()
Range("A1").Select
Selection.NumberFormat = "yyyy mm dd" 'Date as10/06/2005
Selection.NumberFormat = "yyyy/mm/dd" 'Date as 10-Jun-2005
Range("D1").Select
Selection.NumberFormat = "@" 'Text
End Sub