Inactivating the tasks means 'cancel' a task or group of tasks, while keeping original record of the tasks within the project plan.
On the View tab, in the Task Views group, click Gantt Chart.
Select the number of tasks which you wish to inactive.
Click inactive, on the Task tab, in the Schedule group. Text of inactive tasks become dimmed and has a 'strikethrough' line present through across the tasks information.
To reactivate an individual task or group of tasks, select the inactive tasks, and then click Inactivate.