'X-Frame-Options' to 'deny' issue with Force.com site

1. Go to Sites under Setup.

2. Under Sites section, click the Site Label to open the Site.

3. Set Clickjack Protection Level to "Allow framing by any page (No protection)".
Note:  It takes at least few mins to reflect the changes. So, test it after 5 mins to view the changes in Incognito Mode.