No. If you download a file from Project Insight and make change, you must upload that changed file back into Project Insight.
When you hit "download" you are saving that file to a temporary file location (usually the downloads folder). Any saving you do on your desktop computer is saving the changes to that file in the downloads folder - not Project Insight.
If you want to see the changes reflect in Project Insight, follow the same steps as when you upload a new file. The results will be the replacement of the existing file with the new one (if version control is enabled it will also save the previous upload as a separate file).