//where [1] is column number which you want to make text
ExcelWorksheet.Columns[1].NumberFormat = "@";
//If you want to format a particular column in all sheets in a workbook - use below code. Remove loop for single sheet along with slight changes.
//path were excel file is kept
string ResultsFilePath = @"C:\Users\krakhil\Desktop\TGUW EXCEL\TEST";
Excel.Application ExcelApp = new Excel.Application();
Excel.Workbook ExcelWorkbook = ExcelApp.Workbooks.Open(ResultsFilePath);
ExcelApp.Visible = true;
//Looping through all available sheets
foreach (Excel.Worksheet ExcelWorksheet in ExcelWorkbook.Sheets)
{
//Selecting the worksheet where we want to perform action
ExcelWorksheet.Select(Type.Missing);
ExcelWorksheet.Columns[1].NumberFormat = "@";
}
//saving excel file using Interop
ExcelWorkbook.Save();
//closing file and releasing resources
ExcelWorkbook.Close(Type.Missing, Type.Missing, Type.Missing);
Marshal.FinalReleaseComObject(ExcelWorkbook);
ExcelApp.Quit();
Marshal.FinalReleaseComObject(ExcelApp);