Please try using this Microsoft tool which helps to automatically fix issues that might be preventing you from installing/uninstalling programs. You just need to download the troubleshooter, run it and then follow the steps provided by the tool. After running the troubleshooting tool, please try updating your Viewer once again or installing the latest version from scratch.
I am getting an error message saying that 'viewer6.9.msi' is missing and it's asking me to point to it's location. I no longer have the old 6.9 installer and can't get it to uninstall or update to the latest version.