How to focus or select a cell in Excel

 Excel.Application ExcelApp = (Excel.Application)Marshal.GetActiveObject("Excel.Application");
            Excel.Workbook book = ExcelApp.ActiveWorkbook;
            Excel.Worksheet sheet = book.ActiveSheet;
            Excel.Range ExcelRange = sheet.get_Range("A1");
            ExcelRange.Select();

Here is one way:

Excel.Worksheet activeSheet = ThisAddIn.ExcelApplication.ActiveSheet;
var range = activeSheet.get_Range("A1", "A1");
range.Select();

ThisAddIn is the name of my test project.

To select the A2 cell, for example:

Excel.Worksheet Worksheet = excel.ActiveWorkbook.ActiveSheet;
Worksheet.get_Range([1,2], System.Reflection.Missing.Value).Select();