Reordering posts using drag and drop
All you need to do is hover over the post you'd like to move and you'll see a four-headed arrow appear on the left. Simply drag the post to anywhere you'd like in the queue.
Please note that posts that have been scheduled for a custom date and time cannot be moved using drag and drop since they are locked in place. You should be able to spot these posts easily as they will have a blue banner, highlighting the publishing time. If you'd like to move them, you can switch them to your usual posting schedule by hovering over the blue banner and clicking Switch to Buffer Schedule. You will then be able to move the post using drag and drop.