You have two possibilities:
Check the box before the task to be completed
Click on the task to be completed
From the action bar, click Finish.
Complete multiple tasks at once
It is possible to complete several tasks at once by selecting them.
Click on the first completed tasks, then hold down the Control key (ctrl) on the keyboard
Select the other tasks by clicking on them and release the Control key only after the operation is completed
Click on Done
It is possible to reset a task in progress.