Update a list item using Client Object Model

// Starting with ClientContext, the constructor requires a URL to the 
// server running SharePoint. 
ClientContext context = new ClientContext("http://SiteUrl"); 

// Assume that the web has a list named "Announcements". 
List announcementsList = context.Web.Lists.GetByTitle("Announcements"); 

// Assume there is a list item with ID=1. 
ListItem listItem = announcementsList.Items.GetById(1); 

// Write a new value to the Body field of the Announcement item.
listItem["Body"] = "This is my new value!!"; 
listItem.Update(); 

context.ExecuteQuery();