1. Go to store
2. Buy Bluetooth mouse
Seriously, entire workings of the mouse would need to be swapped out with the guts from a bluetooth mouse. It's like converting an electric stove to run on gas....
Short of adding a PIC to convert USB to bluetooth, adding a bluetooth radio, and writing your own code to handle the translation, as well as adding several batteries to give it a respectable run time, it's not worth the headache. Your best bet is to cannibalize a bluetooth mouse and try to cram it's PCB into the USB mouse.