I'm working with Apache POI 5.4.1

protected void init(final HSSFWorkbook workbook) {
    if (workbook == null) return;
    final HSSFFont font = workbook.getFontAt(0);
    font.setFontName("Calibri");
    font.setCharSet(Font.DEFAULT_CHARSET);
}

I presume Font at zero is default Body style font.