Upgrading:
Wifi Fixer will be routinely updated.
When it is, you will be given a Notification by the Android Market.
The process is identical to any other app, you simply tap Update and the app will download and install.
However, because Wifi Fixer depends on a background service, you must open the app or reboot your phone after updating to start the service.
By Google design, there is no way to have a service automatically start on install.
Important: There's a bug where if resources in the app are changed, odd behavior can result on update. This applies to all apps, not just Wifi Fixer.
If this happens, either uninstall and reinstall the app, or reboot the device.