Change the status of a task
There are 2 possibilities for changing the status of a task.
In the overview of all tasks, on the left-hand side for all task there is a symbol of a check mark that you can click on to mark the task as completed.
Open task details and at the bottom of the details overview there is a button you can click on to mark a task as completed.
By clicking on one of these options, you can set the task back as not completed as well.