After you have opened a file and made changes, you need to save it and upload the changed file to the server.
In Dreamweaver, nothing works properly without a working site definition.
To upload a file or folder, select it in the Site window and click the blue up arrow. Dreamweaver calls this the "Put" button. Use the green "Get" button to download from the server. This will overwrite a selected local file or folder with the one on the server.
Sometimes it is helpful to see the server files. To do this, click the Expand/Collapse button. This will show side-by-side views of files on the server and local root.
In the default view, server files are listed on the left and local files are listed on the right. If you don't see the server files, click the connect button in the toolbar.