hello everyone , can anybody help me to add a download csv button on a suitelet page which will works on a post method means after clicking the the download button the data is fetched from the sublist of a suitelet paget with updated data

Yeah, you have to add logic to parse that sublist to create the csv and then use the file module to create it and then do the download logic.

You can use the native methods to get data from that sublist:

ServerRequest.getLineCount(options)

ServerRequest.getSublistValue(options)