From the console you go to Configure services under the "Confgure" menu, then click the "Inventory" tab, and click the "Software" button. This will call up the "Software Scan Settings" panel. Check the box for "Save history. (days):", and then fill in the number of days back that you want to keep history. After you make the change the inventory service might restart, or might not. I can't remember if it does for this one.
Beyond this, it'll be an SQL edit to blow out any particular records.