You will need to delete the old connection and then create a new connection with the same settings. You can’t update the certificate alone. However the new connection will have the same ovpn settings but just with a new set of certificate files.
Then you will need to replace those certificate files in the client. The ovpn configuration file can stay as it is unless you have embedded the certificate parts into the ovpn file in the client in which case those will need to be updated in the config file.