Starting from TeamCity 3.0 server URL can be modified right from the Administration-> Server configuration page.
In most cases TeamCity correctly autodetects its own URL and uses it for constructing URLs in notification messages. But sometimes it is impossible, for example, the server might be running on HTTP and HTTPS or it might be hidden behind the Apache web server. For such cases you can specify server URL in the main-config.xml file (no server restart is required after the change):