The certificate is only one small part of the move to https. You will need to check for external resources that use HTTP addresses such as CSS, JavaScript, fonts and warehoused images and change them as well (called mixed content).
Active mixed content, like JavaScript, CSS and fonts will “break” your page under HTTPS and not render correctly.
After you have the certificate, fixed the mixed content you’ll need to redirect all your HTTP to HTTPS, usually done with htaccess changes.
There’s lots of posts on this forum on making the switch. The fastest and easiest way would be through CloudFlare, as they take care of all, including mixed content and redirects for you.
Check out this post and the link to the article from CSS tricks.
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