To do that, we need to utilize the memory property of each creep that allows writing custom information into the creep's "memory". Let's do this to assign different roles to our creeps.
All your stored memory is accessible via the global Memory object. You can use it any way you like.
Write a property role='harvester' into the memory of the harvester creep and role='upgrader' — to the upgrader creep with the help of the console.
Documentation:
**Memory object** <<< links to http://docs.screeps.com/global-objects#Memory-object
Creep.memory
Memory object <<< links to http://docs.screeps.com/global-objects#Memory-object
Should be
http://docs.screeps.com/global-objects.html#Memory-object