I need to store a data in to Flash, also I need change (rewrite) the data while application is running and store it again. How can I on-fly programm external FLASH? Pease, advise me.
You will need to read the datasheets and other documentation for your FLASH Chip(s) - that will tell you the programming algorithm. Your Vendor might have drivers and/or sample code, app notes, etc. Note that (most) FLASH chips require a whole sector to be programmed at once.