The only field that can be edited in the form field is the submit button. You can edit or add any text that you want.
You can do this by going to the Theme tab of the toolbar and below you will see the Submit Button under forms. Hit the small edit button to expand the toolbar.
In the screenshots below you can see that the text "submit" on the first image will be replaced with a new text on the second screenshot.
CHANGING TEXT COLOR, TEXT SIZE, BACKGROUND COLOR, HEIGHT and PADDINGS
When editing the submit button, make sure that you are at the Theme tab, scroll down until Button Style.
You will see options to change/edit the text size, color, button background-color, and others as shown in the screenshot below.
Remember, the builder is autosave, and changes reflect immediately so there is no need to look for a save button.