Just so i remember – you can look at and modify the contents of a SharePoint .stp file by saving it and then changing the extension to .cab. You can then open, view and extract the contents of this cab file (by double clicking). The stp configuration will be specified in a manifest.xml file within that cab file (note that there may also be some other files containing content etc.
Once extracted and modified, you can make it back into an stp file by using visual studio to create a cab file project (file -> new -> project -> setup and deployment projects -> cab project). Add the relevant files and build the projects. Once complete simply change the extension of the .cab file to .stp.