how to change itunes media folder location

There is a bug in recent builds of iTunes that, at least for some users, seems to stop the media folder preference from being saved properly. See Make a split library portable - Apple Community and put your library in a shape that isn't affected by the issue, and is easier to backup and move around in future.

tt2

What are the current locations of your iTunes Library.itl file and your media folder? I can reduce the complexity by turning my generic advice into something specific for you. Each step can be undone if it doesn't have the desired result, but if followed correctly that won't be an issue. I know what you want, and that iTunes cannot do it for despite the appearance of an option to do it. I cannot fix the app, I don't work for Apple, but I can help you workaround the issue.

tt2