Try deleting the installation folder, e.g.
C:\Program Files\Mythicsoft\Agent Ransack
If that doesn't work because some files are in use you need to run this command:
regsvr32 /u shellext.dll
you should then reboot and try deleting the folder again.
If the folder deletes correctly you then need to delete the registry key which the installer uses to detect the old install:
Finally, try installing the new version again. Any problems please contact Support.