To complete a Task in the lead or client file, go to the right hand column and check off the Task. Once you've completed a task, it will X it out for a while, then it will go into their COMPLETED TASK list, which you can view by clicking on View Completed Tasks (that option will not show until there is a completed task.
To complete a Task in the Calendar, go to Calendar, List View, Complete or Delete (also, note that you can filter easily by Task, Reminder, Agent, Upcoming, Overdue and more using the filters.
Complete Task in Lead/Client File:
Complete Task in Calendar in LIST VIEW:
*You can use the Complete/Delete options on each task individually or use the Select/Select All to bulk complete or delete.