So, I’ve changed my domain from .cf to .ml but I can’t figure out how do I set my website with .cf domain to use .ml domain
It depends on how you've configured your domains and your website.
If you added the .ml domain as an addon domain, you'll probably need to move the files from the htdocs directory of the .cf domain to the htdocs directory of the .ml domain. If you added the .ml domain as a parked domain, that's not required.
After that, you may need to update some settings or code to make sure your website uses the new URL. How to do that varies completely based on what you used to build your website.