In order to edit the Non-Inventory Item you can:

1. Click the Non-Inventory Item Catalog link, located on the left side of the screen.

2. Click the View Non-Inventory Item Catalog link in the Common Tasks section on the Start Page.

In the opened window, click Edit Non-Inventory Item.

The next window will open. Make necessary changes (e.g. period length - not 6 but 9 months). Then click Save and Close. 

In the next window, you can see the changes.