The code for the contact form is found in the front-page.php file. You can find the beginning of the code with the following:
<!– CONTACT FORM–>
or more directly, the first can be found here:
<input type=”text” name=”myname” placeholder=”Your Name” class=”form-control input-box” value=”<?php if(isset($_POST[‘myname’])) echo $_POST[‘myname’];?>”>
All you need to do is find each of the <input> and change the placeholder.