byte[] excelFileContent = await SpreadsheetRef.ExportToExcelAsync();
XlsxFormatProvider formatProvider = new();

Workbook workbook = formatProvider.Import(excelFileContent);
Worksheet worksheet = workbook.ActiveWorksheet;

CellIndex cellRange = worksheet.ViewState.SelectionState.ActiveCellIndex;
worksheet.Cells.GetCellSelection(cellRange).SetValue("Selected!!");

 byte[] newExcelFileContent;
 using (MemoryStream ms = new())
 {
   formatProvider.Export(workbook, ms);
   newExcelFileContent = ms.ToArray();
 }

 FileData = newExcelFileContent; 
 await InvokeAsync(StateHasChanged);