Things is my primary task manager of choice. One of the many features I appreciate is being able to change multiple tasks at once, and it works on both iOS and macOS.
iOS: In any list, swipe left on a task. This enables a bulk edit interface. Tap one or more other tasks, then use the options below to schedule, move, tag, duplicate, share, and more.
Mac: Like most apps, simply hold Shift or Command while clicking multiple tasks, then use any of the available options from the menu bar.