To modify the rating of the breaker, you need to be in the same .dwg file where the panel resides.
Other than that, just double click the rating value in the Rating column for the appropriate breaker row, and you should be set. You can select multiple rows at once, and change the rating of all rows at the same time.
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.