1) Go to 'Co-workers & Groups' link under 'Admin' on the left navigation
2) Click on the group name on the right. You will see all the agents present in the group in order
3) You can now re-arrange the order of the agents by dragging and dropping them within the group.
Eg: Lets say you have 3 agents in the order 'A', 'B' and 'C'. You can now drag agent 'C' and place it above agent 'A' to change the order to 'C', 'A', 'B'.
After re-arranging, the calls to this group will first got to agent 'C'. If 'C' is busy or OFF then it will go to 'A' and so on.