How do I adjust amount due after the form is submitted?

We are registering people for a conference.  We offer scholarships.  I need to be able to adjust the amount due once the scholarship is approved and send an invoice.  

I believe you are referring to editing the form submissions. It is possible, please refer to this user guide: How-to-Edit-Submissions-Made-on-Your-Forms.

In regards to sending an invoice, you can create an autoresponder email. Here's a guide: Setting-Up-an-Autoresponder-Email. We also have integration with Freshbooks, here's a related blog post: Create-Invoices-from-Forms-Using-Freshbooks-Integration.

Hope that helps. If you need further help, let us know. Thank you.