Periodically, we upgrade the digital certificate in the software that certifies where it came from. The certificate enhances the security of the software from hacking, so it's important for us to keep it up to date. When we change it, you'll receive a certificate warning. When that happens you need to manually uninstall the prior version before install will work.
Uninstall using the Windows Control Panel or on Macs, move the old version to the trash (you'll find it in the Applications folder). Once that's done, re-run the install and it should be fine.
On Macs, sometimes dragging to the trash doesn't quite get all the vestiges removed. You may need to track down a copy of AppDelete to get the rest if you're still having problems.