It is not strictly necessary to uninstall prior to installing the newer or an upgraded version, however the uninstall facility will then only remove files installed with the latter installation. If you have created your own Short Term Plans then its best to be safe and Export to floppy or to your hard drive first. The installation program uses file date to decide on replacement.
The Setup program can repair a damaged installation, update files with earlier version numbers and/or, as appropriate, replace files.
Latest:- if your version is earlier than 4.06 then database changes require the updated structure so uninstall or delete STPlan files.