使用jxl api / Apache POI编辑现有的excel文件

I am interested and would like to learn more about java , how to write into existing excel sheets / manipulating the existing data. I was wondering if you could give me an idea on how to edit an existing excel file and save it using the jxl api / Apache POI or perhaps give me a sample program on how to edit some data in an existing excel file and then save it Thanks in advance !!I am interested and would like to learn more ab