Manage, then Delete Agent, works only when the agent isn't running and has no active plan.
"Stop the agent before deleting it." Click Stop and wait until the status shows Stopped.
"Cancel the agent's subscription before deleting it." Cancel the plan (see How do I cancel my agent plan?). The plan stays active until the end of the period you've paid for, so you can delete the agent once the plan has ended.
Once deleted, the agent, its logs and its transactions no longer open, so download any logs you want to keep first (see agent logs). Details: Delete.