Upgrade
Minor version upgrade automatically happens in the background. You don't have to do anything.
For major version upgrade. You will have to run the installation script of latest version. It will automatically uninstall the old version and install the latest version.
Backward Incompatibality
Major version upgrade are complete rewrite with new structure for better performance and high malware detection. So they are not backward compatible. Old scan results and reports will be gone if you do major version upgrade.