Instructions
Unfortunately, you cannot simply install a new version of RIFT over the top of an old version. If you want to install a newer version of RIFT you must first uninstall the previous version.
Upgrade/Uninstall Instructions
Uninstall the old version via the Windows Add/Remove Programs functionality.
After a short time, the old version will be gone.
You may be prompted to reboot your computer.
Once uninstalled you can proceed with the installation of the newer version.