Editing forms via Form Builder is very simple and easy, to edit a form in your page just click on the Message icon in element's blue bar which contains the form.
Then the settings panel will appear on the right of screen, the settings contains:
Fields: from this tab you can drag and drop new fields to your form.
Settings: from this tab you can choose a confirmation popup, add a redirect link, choose form type (custom email or a specific email marketing service) and enter list ID.
If you choose to add an email marketing service to your form (via settings tab), don't forget also to setup the integration from your project's settings page.
You can check this article How to configure an integration
To see the options of a field in your form, just put the mouse on the fields and the options will appear: