After you added a task to the plan you can easily reassign or reschedule it using a simple drag&drop.
If you don’t know how to add tasks, then follow this guide: Add tasks
After you located the task you want to edit all you need to do is hover over it and grab it with the left mouse button. While you hold the button the task will follow you and you can drop it to any other user and day. The place the task will end up will be highlighted.
If you want to change the duration of the task, for example, to make it 2 days long instead of 1 day then:
Hover over the task.
Grab one of the icons that showed up.
Move the mouse over the desired day while holding the left mouse button.
Stop holding the left mouse button while over the desired day.
The task duration should be changed now.
