You cannot directly edit the plan from Collaborator. You must get in touch with your trading partner and they will amend the plan and perform any required changes.
Within each plan there is a 'Comments' tab which allows you to communicate with your trading partner. The 'Comments' tab is a chat system where you can send and receive messages to and from your trading partner. You can also add attachments to the chat which can be useful to demonstrate your concerns and questions with your trading partner. If the plan needs amending, they can check your comments and make any necessary adjustments in their DealTrack tenant.