Here is InterOp code to rename the specific work book as:
using Excel = Microsoft.Office.Interop.Excel;
object oMissing = System.Reflection.Missing.Value;
Excel.ApplicationClass xl=new Excel.ApplicationClass();
Excel.Workbook xlBook;
Excel.Worksheet xlSheet;
string laPath = Server.MapPath(@"\excel\xl_table.xls");
xlBook = (Workbook)xl.Workbooks.Open(laPath,oMissing,
oMissing,oMissing,oMissing ,oMissing,oMissing,oMissing
,oMissing,oMissing,oMissing,oMissing,oMissi ng,oMissing,oMissing);
xlSheet = (Worksheet)xlBook.Worksheets.get_Item(1);
xlSheet.Name = "MyOwnSheet";
xlBook.Save();
xl.Application.Workbooks.Close();