The Reverse Clips button is available on the ClipHistory toolbar. Click it to reverse the order of the selected clips in the ClipHistory. Selected clips below the topmost selected clip are placed immediately above the topmost selected clip in reverse order.
In the Operation Preferences you can choose whether automatically captured clips are added to the top of the ClipHistory (like a stack growing upwards) or to the bottom of the ClipHistory (like a list growing downwards). With the Reverse Clips command you can switch between the two for clips that have already been captured.