Hello.. I'm playing with the ICSP and It's been working well...but,

If a circuit is supplied with 3V battery, how would be the connection of the ICSP's 5V VDD?
I think that this is not a good idea linking the VDD's wire since they are in a different level.. So, What is the trick in that? A on-off button to change the VDD is necessary in this case?

Thanks in advance.....