Changing the SharePoint Welcome Page Programmatically

CSOM

ClientContext context = new ClientContext(webUrl);
Web web = context.Web;
context.ExecuteQuery();
Folder folder = web.RootFolder;
context.Load(folder);
folder.WelcomePage = "SitePages/MyCustomPage.aspx";
folder.Update();
context.ExecuteQuery();
 
//For other folder (e. g. Site Pages)
ClientContext context = new ClientContext(webUrl);
Web web = context.Web;
context.ExecuteQuery();
List rootFolder = web.Lists.GetByTitle("Site Pages");
context.Load(rootFolder);
rootFolder.RootFolder.WelcomePage = "Forms/AllPages.aspx";//"MyCustomPage.aspx";
rootFolder.Update();
context.ExecuteQuery();

Fuente: http://mypowershell.webnode.sk/news/set-site-collection-welcome-page-by-client-object-model/