How do I change the loss of the top cover or ground?

 Geometry Elements - Metal Types

Defining metal loss of the top or bottom of the six-sided enclosure is a two step process:

1.   Define a metal type. You can either define a metal type using the metal library or you can define a metal type by manually entering the loss parameters.

2.   Assign the metal type to the top cover or the ground (bottom of the box). To do this, select Circuit => Box and select the metal type from the Top Metal or Bottom Metal drop list in the Box Settings dialog box.