It is a bad practice to overwrite your original, unless you have another copy of the original some place else.
But......you can do it.
For free there is Irfanview.
For under $100 there is Adobe Photoshop Elements 4.
Both will let you overwrite the original but they will always ask if you really want to do that.
Just click yes.
I would like to find an easy to use photo editing program that I can edit photos and REPLACE the original photo instead of still having the original and the corrected copy. I have tried 4 or 5 different programs and they all want to save the original plus add the corrected one. This is very confusing when I try to look up the photo there is 2 copies which I dont want or need.