I created an application using the online AppStudio and opened his code using AppStudio on the desktop to make any edits. I wonder how do I get the edits I made the source code to be updated in the application.
What you are looking for is the Upload Tool. Highlight your project in the Appstudio Gallery and Select the Upload icon (cloud with an up arrow).
Log into ArcGIS Online (make sure you have an AppStudio Standard License) and use Update to save your local changes to the copy of your app stored in ArcGIS Online. You can even add a comment to help track the changes you have made to each version.
(Screenshot is in French, my apologies)