I have seen several questions about reversing the order in a slider. The recent questions have not been answered. In 2009 there was a question asked for 2006b, and it was answered that it can't be done, and there are no workarounds.
This isn't a trivial issue, and I am surprised that it hasn't been looked at, particularly with App Designer being a remake of many things. If the slider is vertical, increasing down is certainly a meaningful option, to correlate with adjacent axes. Please let me know if there is hope or a workaround.