When a domain does not have an SSL certificate on the server and an HTTPS request is made to the domain, the SSL certificate that is returned from the server will be the domain marked as the "Primary Website". This is further explained in the following article: Why does HTTPS show the website of a different domain?
This article is intended to show you how to change which website is set as the Primary Website on an IP address.
First login to your WHM as the root user and access the "Manage SSL Hosts" tool located at: WHM »SSL/TLS »Manage SSL Hosts
On this page, you will see a list of all of the domains that currently have an SSL certificate attached to them. To set a website as the Primary Website, you will use the "Make Primary" button available next to the website you want to use.
If your server has multiple IP addresses, you can set each IP address to have its own Primary Website.
Note: In the example screenshot above, all of the SSL certificates are self-signed. It is recommended to use a valid SSL certificate for the Primary Website, such as an SSL acquired from AutoSSL or from an SSL provider.