Наконец-то я решил проблему. =))
SPListCollection collLists = oWeb.Lists; //Get All lists
foreach (SPList oList in collLists)
{
if (oList.BaseType == SPBaseType.DocumentLibrary) //Serach for all Libraries
{
SPDocumentLibrary oDocumentLibrary = (SPDocumentLibrary)oList;
string name = oDocumentLibrary.Title.ToString();
if (name == "LibraryName") //Get library what you need
{
SPListItem row = oDocumentLibrary.Items[i]; //get an element by id in library
SPFieldLookupValue newValue = new SPFieldLookupValue( id , "LookupFieldValue");
row["field_name"] = newValue;