When you are viewing a task in a kanban board or a task board with multiple users’ tasks listed, click on the checkbox to the left of the task's title. This will expand the Task Action Buttons. The very first action button is Mark Task as Complete, click it and you task is complete!

Another option is to double click on the task to open its details. Enter today’s date in the Date Completed field and your task will be considered complete. When your task is complete a post will automatically be made for you on your profile page. If that task is linked to a project, that project will be a few percent closer to completion.