When you use the .sh installer, there is this message:
Do you want a Unix-style distribution?
In this distribution mode files go into different directories according
to Unix conventions. A "racket-uninstall" script will be generated
to be used when you want to remove the installation. If you say 'no',
the whole Racket directory is kept in a single installation directory
(movable and erasable), possibly with external links into it -- this is
often more convenient, especially if you want to install multiple
versions or keep it in your home directory.
ps: I don't think anyone can remember this message exists when they try to upgrade/uninstall racket 4 months after installation. I just stopped using the .sh install for this reason.
You can try the racket-uninstall, or nuke the racket/ directory, then install the new version.
You probably also want to scan invalid racket symbolic links under ~/bin? ~/local/bin/? ~/.bin?