Click on the task on the task list or in the pending tasks list.
Use the "Status" dropdown to change the status and click on the "Save" button to edit the task.
When changing task status to "In progress", you can also fill in the % to indicate progress.
Concerning the completed tasks, you can manually edit the time of completion, e.g. if you completed the task earlier than at the moment when you reported it in your task list.