I upgraded Haxe to version 3.2.1 but there are some bugs cause errors in my project like: https://github.com/HaxeFlixel/flixel-demos/issues/208
How can I downgrade Haxe to a specific version like 3.2.0 without uninstalling / reinstalling?
Go here:
http://haxe.org/download/list/
Depending on your platform (mac/windows/linux) it can be easier or harder, but usually you just uninstall the version you have and run the new installer.
If you're on linux I recommend using one of the haxe install scripts openfl has put together as they're a bit easier to work with than the official haxe installers (but it's the same stuff inside):