Once you complete a task, or need to change the status for any reason, please either click directly on the task itself, or on the blue pencil icon () towards the right of the screen.
You'll then see this screen
As you can see, the status of the task is 'Open'. To change that, simply utilize the Status dropdown menu to change it from 'Open' to 'Complete' for example:
If you go back to the task center homepage, you'll notice the 'Review Blood Work' category's status is now 'Complete'. You have the option of selecting either 'Open', 'On Hold', 'In Progress' and 'Complete':
To view how to create a category, please click here