The RS485 specification is only the definition of the "electric level"
How you addres units and how you exchange information between them is up to you to implement.
The usual way to do it is simply to let each unit have a one byte addres.
this could be defined in the software, or you could do it with a 4 (or bigger) position dip switch hooked up to some digital pins on Arduino.
A good example can be found here:
Another way is to check one of the "real" implementations like Modbus, good introduction here: