And I just tested. The only way to get it to stop asking to upgrade is:
Run the app from the desktop, when it asks to upgrade, decline.
Click the 3 vertical dots at the top right, uninstall C3.
Open the browser, go to editor.construct.net.
Again, it prompts to do an update. Select yes.
Then, from the C3 menu, install the app.
If I run the app from the desktop and choose yes when it asks to upgrade, it never gives me the option to upgrade or install the app. I have to uninstall it first.
It's really no big deal to just keep clicking on the 'upgrade' button. It was just confusing as I expected an upgrade and not a redirect.