How can I edit the fields after I post them?

I'm not sure if I understood it correctly, do you want to edit the form fields? Or do you want to edit the submission data?

You can simply edit the form by opening it on the form builder.

If you want to edit the submission, then you can do that in your submissions page, you can click the "Edit" button there. 

In edit mode, please note that you need to click the "Submit" button for the changes to be saved in that submission.

If I misunderstood your concern, please do provide us more details of what you would like to accomplish.