You can mark a task as "Completed" by clicking directly on the "Validation" icon in the bottom right corner right of the task.
You can also specify a percentage of progress other than 100% (task completed). To do this, go to the detail of the task panel by clicking on the relevant task, go to the "What" tab and then move the cursor set to 0 by default on the progress bar.
The percentage of progress is displayed on the task in the What tab/view.
Note: Use the filters in the top right corner to filter the display of your tasks by their status.