You can only reorder your tasks based on Due Date, Name or Priority when you are in your Projects view (as opposed to a Daily or 7 Day View). To reorder the tasks, click on the Gear icon at the top of the list that is in the right-hand pane and select to sort by (Due) Date, Name or Priority (there are also shortcuts available).
This hint is applicable to the web version.
The "Reorder" button you see at the bottom of the task list in some older videos has been removed. Instead, we've introduced the drag & drop option allowing you to reorder tasks anytime.
Please take a look at this short video to see how the new option works: https://www.youtube.com/watch?v=lhumUr8kY6I