Please check the demos source code after you install Aspose.Cells. And you can check online wiki docs at http://www.aspose.com/wiki/Default.aspx/Aspose.Cells/ .
I looked at the online samples and all that they do is they take one file as reference and write data to it and save as another file, but what I want to do is open an existing file write to it and save with the same file name.
You can do it in the same way. Just specify the same file name while saving file.