If you no longer need an SSL certificate on some domain, you can uninstall it by navigating to Hosting → Manage, search for SSL on the sidebar and click on it:
There, locate the domain and click on Manage:
And click on the options button (⋮) → Uninstall:
That's it! Now you can install a different certificate on the domain, such as a custom SSL.
After the SSL certificate is uninstalled, your website will only be accessible by HTTP. Trying to load it as HTTPS will show an error
If you would like to temporarily disable HTTPS to have your website available by HTTP, you can unforce HTTPS