Change CA for client certificates

[theoneyouknow]

Hello,

what do i have to do to change the CA certificate for the clients?
The description here only works for the admin/login gui and there are no further informations on how to change the client ca.

I'd like to use a standard ssl certificate for admin/login gui (already working) and my own ca for the clients within the .ovpn files.
Whatever i try - the clients still use certificates issued by standard OpenVPN CA.
Any ideas?

Thanks