How to modify photo time stamp?

[wc2005]

1. When copying photos from phone/PC to my phone, the photo time stamp will be changed to the current date time of the phone which is incorrect.
2. The Photo application groups photos by time stamp, so the only way to regroup the photos is modify the time stamp.
Does any body know how to modify photo time stamp or an application to do that? I'm on Windows Phone 8.1 update 2. Thanks.