Here I show you all code to set excel font:
sheet.Range["A3:E3"].Style.Font.FontName = "Comic Sans MS";
sheet.Range["A2:E2"].Style.Font.Size = 50;
sheet.Range["A3:E3"].Style.Font.IsBold = true;
sheet.Range["A3:E3"].Style.Font.Underline = FontUnderlineType.Single;
sheet.Range["A4:E14"].Style.Font.IsItalic = true;