If you want to change the associated contenttype for a pagelayout, you can do it like this:

// first you need a reference to a PublihingSite or PublishingWeb
var pubSite = new PublishingSite(site); 

// then we need to get all available pagelayouts
var pageLayouts = pubSite.GetPageLayouts(false); 

// try to find the pagelayout you want to update
var pageLayout = GetPageLayout(pageLayouts, "pagelayout.aspx"); 

// if pagelayout exists, update it (here you need to have a ref to the contenttype)
if (pageLayout != null)
    UpdateAssiciationContentType(contentType, pageLayout); 

/// <summary>
/// Helper method to update associated contenttype for a pagelayout
/// </summary>
/// <param name="newsContentType"></param>
/// <param name="pageLayout"></param>
private static void UpdateAssiciationContentType(SPContentType contentType, PageLayout pageLayout)
{
    // Check out the pagelayout
    pageLayout.ListItem.File.CheckOut(); 

    // set AssociatedContentType
    pageLayout.AssociatedContentType = contentType; 

    pageLayout.Update(); 

    // CheckIn, Publish and approve changes
    pageLayout.ListItem.File.CheckIn("Changed associated contenttype");
    pageLayout.ListItem.File.Publish("");
    pageLayout.ListItem.File.Approve("Change approved");
} 

/// <summary>
/// Get pagelayout by name
/// </summary>
/// <param name="pageLayouts"></param>
/// <param name="name"></param>
/// <returns></returns>
private static PageLayout GetPageLayout(IEnumerable<PageLayout> pageLayouts, string name)
{
    var layouts = from PageLayout layout in pageLayouts
                  where layout.Name == name
                  select layout; 

    return layouts.Count() == 1 ? layouts.ElementAt(0) : null;
}

If you're looking for a specific item in a list, there are many ways you can get it.

You could go through the whole list using a loop (not recommended):

foreach (SPListItem item in list.Items) 
{ 
    if(item.Title == "Title1892") 
        return item; 
}

You could use a SPQuery:

var tmpQuery = new SPQuery 
                   { 
                       Query = 
                           @"<Where> 
                              <Eq> 
                                 <FieldRef Name='Title' /> 
                                 <Value Type='Text'>Title1892</Value> 
                              </Eq> 
                           </Where>" 
                   }; 
list.GetItems(tmpQuery);

or you could use LINQ:

Method 1:

List<SPListItem> q = (from SPListItem item in list.Items 
        orderby item.Title 
            ascending 
        where item.Title == "Title1892" 
        select item).ToList();

Console.WriteLine(q[0].Title);

Method 2:

var query = from SPListItem item in list.Items 
                      orderby item.Title 
                          ascending 
                      where item.Title == "Title1892" 
                      select item;

Console.WriteLine(query.ElementAt(0).Title);