How can I modify a predefined aging method?

You can't modify the predefined aging methods. However, you can copy the predefined aging method to create a duplicate and then modify the duplicate aging method.

To modify, go to the Manage Aging Methods page, select one of the delivered aging methods and click the Duplicate button. By default, the duplicate aging method is renamed with copy as prefix. For example, 5 Bucket Aging becomes Copy 5 Bucket Aging. Make the required changes and save the duplicate aging method.