To select multiple tasks in a project, simply press and hold CTRL (on Windows) i.e. CMD (on Apple) and click on a task. Keep the key pressed and click on a second task. Once you've selected two tasks, little checkboxes will appear in the tasks' upper left corners. At this point, you can let go of the key and use your cursor to check or uncheck boxes of tasks.
When you’ve selected all your tasks, click on the Bulk Action button that has appeared in your top bar to select an action. You can move, duplicate, complete, make active, archive or delete your selected tasks.