When you assign a task in Outlook, the person to whom it was assigned takes ownership of the task, and has the choice to decline or complete it. This means that to cancel a task that you've already assigned, you must do it before you send the task. If you've already sent the task to the recipient, ask him to decline the task so that you can cancel it rather than reassign it.
Launch Outlook, and then click the Tasks icon on the sidebar. It looks like a clipboard with a check mark in the middle.
Double-click the task you want to cancel.
Click "Cancel Assignment," and then click "Save and Close."
Information in this article applies to Outlook 2013. It may vary slightly or significantly with other versions or products.
Canceling a task assignment won't remove it from the list of active tasks. To delete the task, click "Delete Task" after you've canceled the task assignment.