Q: cannot upgrade to 4.2.1

What OS are you running? Is any other version of Compressor installed on any mounted volume?

Russ

What happens when you click on the Apple logo > App Store upper left in the Finder toolbar?  Any updates available?

Russ

Just to verify, Compressor does appear on the Purchased list of the App Store after you have signed in with your Apple ID?

Russ

Then I suggest that you uninstall your existing installation – using this application from Digital Rebellion. Then download and reinstall the application.

But before you do this, do a couple of things. If you have any custom settings you want to preserve, open the Home library/Application Support/Compressor/Settings and copy any of those settings to a folder outside Application Support. Then in Applications/Compressor, right click the bundle and choose Compress Compressor to create a zipped version. That way, you'll have an easy way to revert back to 4.2 if something goes awry in the reinstallation process.

Russ

FCS Remover will find and delete any other Compressor-related files.

If you don't care to use it, sure. Drag the dot app file to the Trash (after you've zipped it, it you follow those steps).

Russ