You can customize the look of the form with custom CSS. You can add that CSS to your theme stylesheet, or to your custom CSS editor available under Appearance > Edit CSS in your dashboard.
If you’re not familiar with CSS, you can check this beginner tutorial. If you’d like, I can also give you the code you need if you let me know your site URL, as well as the changes you’d like to make!
To change the text of the email input field, you can define a custom placeholder text in the widget options, under Appearance > Widgets.