Changing the datasource is really no different from changing any other site configuration. As this is the most frequently made change, however, it has a dedicated page.
As with other site configurations, this is found in "/_config/config.cfm".
Find the following line of code:
setSetting("datasource","");
Then change the value to the name of the datasource. For example, if you datasource was named, "Sample" then you would change it to the following:
setSetting("datasource","Sample");
As the value for "datasource" is a simple value (and therefore contains no components), this is all you need to do. Neptune will pick up the change on the next page load.