To select multiple tasks simply press and hold ctrl/cmd and click on a task tile. After the second selection the little checkbox appears in its upper left corner, and you can click on it. Once your second task is selected, all other tasks will display checkboxes as well.
When you’ve selected all your tasks, click on the new bulk action button that has appeared in your top bar to select an action. You can move, duplicate, complete, archive or delete your tasks.