Do you have a License key? 24 digit numerical? If yes uninstall to existing version of Lightroom using your operating system procedure, this will not interfere with your catalog, preferences and personal files.
Download the appropriate version for your operating system from this site. Product updates proceed to install and enter your License key.
If you have a Adobe Creative Cloud subscription then you may need to uninstall the existing version the reboot and open the Adobe Creative Cloud Application Manager, login to your account and Lightroom CC should be available to install. The application will be automatically activated.