Generally for questions about Windows configuration, you can ask on Super UserGenerally for questions about Windows configuration, you can ask on Super User.
You can scroll to the bottom of the Default apps window and choose "Choose default apps by file type" then scroll down to UnityPackage and click "Choose a default" - then pick your Unity version of choice.
If you can't get Windows to recognize the Unity package type, then you can also open your Unity project and pull the package in with Assets > Import Package
