For some reason, if an upgrade of an AppAssure/Dell Rapid Recovery Agent is not succesfully completed or cancelled, a subsequent upgrade may fail with the following error message:

Error
Destination path is not empty. Please choose another one.

If an upgrade was cancelled or has failed for some reason, the following upgrade attempt will try to perform a complete installation. Because not all files and/or folders have been removed, the installer will not continue and will display the error message.

To resolve the problem, manually remove the files and folders, but leave any registry keys intact to ensure a proper upgrade without losing any settings.

If it is running, stop the Agent or Core  ...

Dell has changed the name of its acquired AppAssure to Dell Data Protection Rapid Recovery. AppAssure (latest version is 5.4.3) is no longer being maintained, so upgrading to Rapid Recovery (latest version 6.0.2 at the time of writing) is a requirement for continued support. Fortunately, performing the upgrading is not really difficult, and can be done without too much downtime for your backup.

Additional information, including a compatibility and upgrade matrix can be found here:

How to Upgrade AppAssure 5.4 to Rapid Recovery 6.X (181997)

Dell Support Policy (compatibility matrix for Core and Agent versions)

At the time of writing, the compatibility matrix shows that Agent version 5.4.3  ...