If you wanna use Javascript then you can use getElementsByName() method to select the input fields and to change the placeholder for each one… see the below code…
document.getElementsByName('Email')[0].placeholder="new text for email";
document.getElementsByName('First Name')[0].placeholder="new text for fname";
document.getElementsByName('Last Name')[0].placeholder="new text for lname";
Otherwise use jQuery:
$('input:text').attr('placeholder','Some New Text');