I includes a existing workbook that contains multiple sheets. I've got a sheet which consists of some bold cells inside it. I wish to alter the font of the entire sheet retaining the boldness of a few of the cells. Are you able to please provide me the code snippet?

You'll have to create several style for that workbook:

XSSFWorkbook wb = new XSSFWorkbook();

XSSFCellStyle fontStyleBold = wb.createCellStyle();
XSSFCellStyle fontStyleNormal = wb.createCellStyle();

and assign cells you would like.