Mark a Task as done from the Planner right-side panel
Option 1
Find the finished Task from the Planner right-side panel.
Click the checkbox to the left of the Task's name.
Option 2
Click the Task to open the Task Policy details view.
Click the check mark at the top of the Task Policy details view.
Mark a Task as done from the Planner calendar
Find and click on the associated Task Event for the Task to open the Task Policy details.
Follow instructions for Option 2 above.