How to Set Pivot Table Cell (.xls) text from C#.Net

Question

I have one XLS, which has one Pivot table with 10 Columns in 4 row,
I am tring to set Cell[4,10]="Somhing" but not able to set text.
I am tring to use
((PivotTable)_ExcelSheet.Pivotable["Pivotable1"]).PivotFileds[10].Caption etc
but it is not giving reference to table.
So i want to get reference of Pivot Table in First Worksheet and want to dynanically set Caption/Text in C#