sorry about the poor experience, you seem to be on windows 10 mobile Insider- Release Preview Ring. you could tell us the right OS build as the one mentioned by you ( preview build 15024.38 ) isn't correct.
First of all, you dont need insiders to upgrade to windows 10 as Microsoft has made the lumia icon eligible for windows 10 mobile upgrade using the upgrade advisor. Also yes you could roll back to wp 8.1 by using
Windows Device Recovery Tool ( W.D.R.T.) , You only need to follow the in-app commands and your device will be flashed to the latest software available on servers.