How to: Set an Active Worksheet
To set an active worksheet within a workbook, assign the corresponding Worksheet object to the WorksheetCollection.ActiveWorksheet property.
// Set the second worksheet ("Sheet2") as the active worksheet.
workbook.Worksheets.ActiveWorksheet = workbook.Worksheets["Sheet2"];
The SpreadsheetControl.ActiveSheetChanging and SpreadsheetControl.ActiveSheetChanged events are raised before and after the active worksheet is changed, respectively.