The reality is that Form Builder works much better as a form "Builder" than it does as a form "Editor", so whenever possible it is usually best to make edits by hand.
It does have a way to re-enter the interface, but at the end of the process it will essentially delete the form and re build it and won't really be editing so that any updates you made will be overwritten by the edit process.
Having said that you can usually highlight the form tag in design view and re-open the form builder interface from the property inspector edit button that should appear.