Editing the materials known to Cimatron is done with the Tools> Material Library function.  This opens up a table.  

You can also access the same table from the Cimatron browser, using the icon right on the material row.
By right clicking along the left edge of the table, you can insert or delete rows.  And by double clicking in the cells you can
edit the values.

*Note: If you set up a row with the headers of other existing attributes like hardness or heat treatment, then changing
the material will also change these values at the same time.