TableCellProperties c = doc.MainDocumentPart.Document.Body.Elements<Table>().ElementAt(0).Elements<TableRow>().ElementAt(5).Elements<TableCell>().ElementAt(2).TableCellProperties;
TableCellBorders tcb = new TableCellBorders(new TopBorder() { Val = BorderValues.None });
c.Append(tcb);

TableBorders tb = 
    new TableBorders(
        new TopBorder() { Val = new EnumValue<BorderValues>(BorderValues.Single) },
        new BottomBorder() { Val = new EnumValue<BorderValues>(BorderValues.Single) },
        new LeftBorder() { Val = new EnumValue<BorderValues>(BorderValues.Single) },
        new RightBorder() { Val = new EnumValue<BorderValues>(BorderValues.Single) },
        new InsideHorizontalBorder() { Val = new EnumValue<BorderValues>(BorderValues.None) },
        new InsideVerticalBorder() { Val = new EnumValue<BorderValues>(BorderValues.Single) }
    );