Thanx, Erik.
But I dont want user to to switch anything in hardware. The idea was to have an option "upgrade" in VB application. PC send the proper command to uC which starts the segment of its own code executing the writting the new version in own flash. Hex/binary file goes from PC via serial port and UART of uC. No JTAG now, uC is just initially programmed with JTAG. Is this possible?