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 at least two tasks, a 3 ellipses button will appear at the top beside the project name. You can deselect a task the same way you select it.
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 Set/Clear Due Date, Add/Remove Tags, Add/Remove watchers, Move, Duplicate, Complete, Archive, Trash or Print the selected tasks.