How to change Vendor/Product ID when compiling bootloaders

I have a Micro also coming in the mail to test and try to understand this. In that case, which is the VID/PID that the computer you attach to is given? (or does that answer differ at different 'levels'). I'm interested in both a 'hardware' level (maybe what your computer shows in the device enumeration), and a 'software' level (what an application is given) [but not sure if i'm talking sense]