To replace the default form on the Contact page with a custom form, navigate to the "Contact" page. 

Then you want to go to the page editor for the contact page and remove the default form shortcode [contact-form].

Next you'll want to click the Add Form button, which is located right next to the Add Media button right above the editor.  

From there you can build a custom contact form.  We have a demo video of how to use the custom form builder here: