If you want to have multiple PixInsight versions on your machine you can rename the PixInsight directory to something like PixInsight-1.8.9-1 and then install a new version (like 1.8.9-2). After that you can switch between versions by renaming the version directory you want to use as PixInsight. It has worked for me in Windows but my understanding is that it should work on other platforms too.