It\'s not possible, though you could make your program reboot in to the bootloader like ducky said. The digispark arduino software doesn\'t have any built in feature to send a \'please reboot\' command to already booted digisparks though.
I think it\'s better this way - unplugging and plugging back in is a nice clear way to specify which digispark it is you want the program to upload in to, if you have more than one on your computer (I have three already!!)