From the desktop or web app:


Hold down "Alt" while clicking on the tasks you want to move. Once you've selected all the tasks, click on the "X selected" button that appears in the top right hand corner and choose "Move Tasks to List".


From the mobile app:


Tap "Edit" in the bottom right hand corner of the list. Tap on the tasks you want to move, then tap "Move" in the bottom left hand corner. Select the list you want to move to, then tap "Move Tasks".