What is the code to select a response on a dialogue box?

I have a Macro which saves the open work book to a couple of locations
- in one of the locations it needs to be saved with the same file name
each time.

Obviously each time the macro runs, Excel opens a dialogue box asking

"Are you sure you want to Save? - this file already exists"

What code can I use to select the Yes button from the Macro?

Cheers,

Steve