How to reference a specific instance of an object created by a function? Obj-C

How to reference a specific instance of an object created by a function?
Obj-C

Good day all, I'm pretty confused on how to, for instance, change a
property of an object created by a function. By calling the function
multiple times I created multiple instances of the same class with the
same object name, now I want to change some properties of a single
specific object. I guess a little background would be helpful for you
folks: I started to play around in the Cocos2D Chipmunk Engine for the
iPhone and in the Example project I can create a character on the screen
multiple time by just calling a method. I want to control the forces
applied to only one specific character. If you need a more specific
example please ask! Thank you =)