To change the agent of an order first search for the Order. Click on the header and select the new agent from the dropdown.

If the order is already in AR or is in a split group, you need to click the red "A/R" button and if it's in a split group you'll need to extract it first by clicking the "Extract from split group" button

You'll then be able to click the agent dropdown and change the agent.

After you correct the agent, click the green Force Finish button.