Follow this procedure to change a site's IP address:

1. Log on to your WHM as root.
2. Go to "Account Functions" section.
3. Click on "Change Sites IP address".
4. Select the account from the dropdown list.
5. Click on "Change".
6. Now you will see a new page with all the IPs available. Select the IP from the drop down list.
7. Click on "Change".

Check the relevant screen captures: