As we all know, there is no way to edit the text in the sign-up form fields. The only options we have are Full Name, Email Address, and Phone Number.
Though you cannot change the text in the form field, you can adjust the text height, text color, the field height, and background color. All these can be done under the Theme tab > Forms > Input Style.
Remember, the builder is autosave, and changes reflect immediately so there is no need to look for a save button.