I was looking for a way to update the page title of my welcome page in c# and I got to the following example:

SPWeb web = SPContext.Current.Site.OpenWeb();
PublishingWeb pubWeb = PublishingWeb.GetPublishingWeb(childWeb);
SPFile defaultPage = pubWeb.DefaultPage;

defaultPage.CheckOut();
defaultPage.Update();

defaultPage.Item["Title"] = "My title";
defaultPage.Item.Update();

defaultPage.CheckIn("My comment");
defaultPage.Update();

Have fun!