You can Reset or Reactivate tasks in the Management tab for a specific Contract in the Navigation Tree.
Reset Task: Will re-set the task. If a task is planned it will Un-plan it and if the task was completed this will un-complete it.
Reactivate: Will create an unplanned copy of the task (this will keep a historic record of the orignal task) .You will have to go back to the task that has been reactivated, and accept or delegate it to a user.
The percentage completed bar for the contract will get updated shortly after the tasks are reset and are processed in the database.