In SharePoint, you can change the default page of a site in two ways:
- if the site is a PublishingWeb, cast you SPWeb to this type and set its DefaultPage property;
- else, change the WelcomePage of its RootFolder:
SPFolder rootFolder = spWeb.RootFolder;
rootFolder.WelcomePage = "SitePages/MyHome.aspx";
In the documentation pages PublishingWeb.DefaultPage Property and SPFolder.WelcomePage Property there are both documentation and samples.