In order to change the item description that appears on the PayPal page, please:
1. Hover over your form and click on the "Payment" icon
2. Click on the green "Assign Prices" button
3. Add your custom item description into the first field of the Fixed Amount property as noted in the screen shot included below:
4. Click on the "Save" button, and then test you form. The form submitter will now be able to review your custom item description on the PayPal page as noted in the screen shot below: