Here are some suggestions for how to respond to your manager's email in a calm, professional manner:
Dear [Manager's Name],
Thank you for the additional update. I'm currently juggling several high priority projects and want to make sure I understand exactly what is being requested here. Could we schedule a short meeting later today or tomorrow morning to discuss this new task and clarify expectations?
My team is working hard to complete our current project on time. Additional responsibilities will require adjusting some deadlines or priorities. An in-person discussion will help ensure we have a shared understanding and the best path forward.
Please let me know what times work best for you. I appreciate you keeping me informed as needs and timelines shift