To search for any Task that currently exist in your database, go to Profiles > Tasks. You can search by users associated with the Task, Task Title, or Due Date. In the results grid, you can edit or delete any task, and mark it as complete. You can also add a new task (linked to a profile you search for, or unlinked to any profile) with the Add a New Task button. Click here for more details on creating a task.
To mark a Task as complete, simply click the Done checkbox directly in the grid. The Task will update as "completed" and use the current date as the Date Completed. If you need to change the Date Completed to a different date, you'll need to use the Actions dropdown to edit and save the date.