You will have to do the following steps if you want to overwrite the default contact form by a form created by xform.
Please navigate to X-Form and click the Cog icon of the form you want to overwrite with.
Now click the ShortCode tab and copy the iframe URL of the form
Now navigates into Extension-> Modules. Install the HTML Content module if it is not yet installed. Now click the Edit button of the HTML Content module
On the Editor, please CodeView icon
Now add the following code i.e a frame and some CSS so it can make hidden default form. Please adjust the height of the iframe accordingly. Also, don’t forget to update the form URL that you copied earlier. Now click the Save button to save the HTML content module.
Now navigate to Design -> Layout, Now edit Contact Layout. Finally, add the HTML content module to the Content bottom position.