I use the Synapse routines GetHTTP to return a specific resource, and if found then check against the local system to see if an update is required. If so then the resource tells me what page to go to launch and I throw the URL into shell execute so the users
preferred browser is displayed.
Most of the time the download is a setup program created by InnoSetup which updates the users system and database to the latest version. When a new "paid" upgrade is needed, I then send the user to a "purchase upgrade" form. My web resources are ASP pages, so I can redirect to a different resource based on the customers version number.
For the main application (our application has a server piece, and a client piece) I have a loader which will check the server to see if the version of the client file on the server is different than the version on the client...if so, it prompts the user if the user wants to update/revert. We chose to prompt the user as sometimes an accidental bug might make it into the system and the user has to downgrade/upgrade only specific machines to help troubleshoot. I maintain a database record with the minimum version required which is updated via the database patch, so if a version must be retired then the record is updated accordingly.