Just noticed the upgrade description on https://compositec1.codeplex.com/releases/view/122260:
Upgrading from earlier Composite C1 4.2 releasesThe package is not available in Composite C1 4.2 Beta. Please check.
If you are among the awesome people who helped us test the beta releases of 4.2, you get the easy upgrade free of charge:
Make a backup of your website.
In the C1 Console, go to "Packages" on the System perspective
Browse down "Composite.Updates" and click "Package Info" on the 4.2 upgrade
In the Package Info view, click install
Complete the wizard
Enjoy