Here's a quick code snippet for updating user profile information in SharePoint
SPSite siteCollection = null;
SPWeb web = null;
siteCollection = SPContext.Current.Site;
web = siteCollection.RootWeb;
web.AllowUnsafeUpdates = true;
SPList list = web.Lists["User Information List"];
SPUser user = web.SiteUsers["DOMAIN\user"];
SPListItem item = list.Items.GetItemById(user.ID);
item["FirstName"] = "John";
item["LastName"] = "Smith";
item["WorkPhone"] = "55378008";
item["Office"] = "BigScholar.com";
item.Update();
The List name needs to remain as "User Information List".