You can Reset or Reactivate tasks in the Management tab which is under the Contract in the Navigation Tree.
Reset will set the task back to unplanned, Reactivate will create an unplanned copy of the task (this will keep a historic record of the orignal task)
There it will have the below screen and you can choose which task to Reset. This will however not update contract status so that will have to be done if required in the Management tab as well. The percentage completed bar for the contract will get updated shortly after the tasks are reset and are processed in the database.