When you complete a task, simply click the checkmark icon next to the task title.

This action will mark the task as closed, turning the icon green and striking through the task title to indicate its completion visually.
​

If you need to see a list of all closed tasks, use the Filters to create a "Closed tasks" filter list, making it easy to review completed work.
​

If you need to reopen a task, simply click the checkmark icon again to unmark it, which will reopen the task. Alternatively, you can update the task's workflow status to any of the active options from your list.

For details on how to close multiple tasks simultaneously, check out this article.