In CRM > Tasks, there are now three buttons at the top of the list which can filter your Tasks so that you can easily find Tasks that are Overdue or Complete.

When you click on Overdue, the list will show only those Tasks which are overdue and not yet complete. To complete the Task, you can click on the tick on the right hand end of the task.

If you are using the card view for Tasks, then the 'Complete' tick will be in the upper right of the card, inside a circle.