I tend to think of the forms module as a one-way input of form data such as surveys or contact, not really intended to be edited. This is not to say it can't or shouldn't be done, but rather that it makes sense to me personally that such functionality is not available out of the box.
However, it certainly can be done, but does require that you create a custom user control form (ascx) with input controls, and use the Sitefinity Forms API
to populate it with existing content and again to post those changes back to the form.
It certainly involves some doing, and I would suggest that Telerik consider creating a PITS item to see if other customers might find such a feature valuable enough to implement. But in the meantime this should allow you to accomplish what you're attempting.
I hope this is helpful