To install the SSL Certificate through your WHM you will need to log into WHM first.

You will want to remove the current SSL on your site first.

Log into your Dedicated Server through WHM
Scroll down to the Web SSL/TLS section
Click on the Delete SSL Host
Remove the existing SSL Cert (if one is installed)

Next click on the Install a SSL Certificate and Setup the Domain
Enter the CRT file that you received when you purchased the SSL Cert
This will populate the Domain Name and the IP number (usually)
You will have to enter the USER field (cpanel username)
Enter the KEY information, if it doesn't automatically populate.
In the last box, enter the CA Bundle file which comes with your Certificate

Scroll back to the top and click "Submit"

This will bring a new screen in which it will authorize the Certificate and then return an "OK" for the installation.

If you have any questions, please contact your Server Management Company, or you may enter a ticket with Hands-on for limited assistance with this issue.