How can I set date to existing excel date cell using java

Question

Use Apache poi api

Answer 1

I'm not sure if I understand what you're trying to do.
To parse a date you get the CellValue. The CellValue class provides the getDateCellValue method.
The next step is to evaluate the date. You could use a SimpleDateFormat or DateTimeFormatter that matches the CellValue formatting and then create a Date: Date date = dateFormat.parse(myCellValue);