Hi,
Select all the images that you want to make the changes to and right-click on the details pane at the bottom of the page and then click on ‘Remove properties’. Then select ‘Remove the following properties from the file’ and select ‘Date taken’ from the list. This will remove the date taken property.
You may also refer, ‘Change the date a picture was taken’.
http://windows.microsoft.com/en-US/windows7/Change-the-date-a-picture-was-taken