To install a SSL certificate, it must match the CSR generated through the Winhost Control Panel. If the CSR is accidentally regenerated after placing a SSL certificate order, it will no longer be possible to install the certificate through the SSL Manager. However, it is possible to rekey / reissue the certificate with the newly generated CSR. Then install the reissued certificate through the SSL Manager in the Winhost Control Panel.
If the SSL certificate was purchased through another certificate authority, generally a quick Google search for reissuing a SSL certificate through the applicable certificate authority will provide a link to the steps.