How can I uninstall these and then reinstall Imatch 2018?
You want to uninstall what?
The message in your screen shot shows up when you try to open a database that has been created by a newer version of IMatch with an older version of IMatch.
Did you uninstall a new version of IMatch and install an old version?
Tip: If you install the latest version of IMatch (2018.9.2 currently) it replaces whatever version of IMatch currently installed on your system.
The message you show is usually only displayed if you have different versions of IMatch installed on different computers and you try to open a database created in a newer version. Nothing you usually encounter.