How to get excel file from sharepoint library and write to it programmatically

we would like to

• read an Excel template stored in a SharePoint 2010 library
• create a new Excel file from it
• fill the new Excel file with data programmatically

we tried already Excel Services but however the given MSDN example seems only to work while debugging…!?

what the best practise for the Szenario?
better using cson or serverside ?

how-to-get-a-file-using-sharepoint-client-object-model-with-only-an-absolute-url

Any help appric.
Thanks.