EDIT: Oops, I misunderstood your question, sorry! I thought you were wanting to change individual instance variable data on an event. I'll leave the post here though in case that particular tidbit of information helps someone, .
You can target specific instances and set the variable data using UID.
** Make a global variable called objCollidedWith **
Player -> On collision with object
-- objCollidedWith = object
object-> Pick instance with UID objCollidedWith
-- sets variable for this instance only