You got to find the subtotal row or range of cells involved, create the style and set the required attributes with relevant style flag options on, finally apply the style to the range of cells or the row accordingly. See the sample code below for your reference:
Aspose.Cells.Cells cells = workbook.Worksheets.Cells;
//Create the range of cells.
Aspose.Cells.Range range = worksheet.Cells.CreateRange(“A5”, “D5”);
range.Name = “Range1”;
Aspose.Cells.Style style = workbook.Styles[workbook.Styles.Add()];
style.Font.IsBold = true;
StyleFlag styleFlag = new StyleFlag();
styleFlag.FontBold = true;
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