The following command will modify the Syslog server entry on all hosts in the vCenter server that the PowerCLI session is connected to.
Set-VMHostSysLogServer -SysLogServer "servername.fqdn.com:9999" -VMHost "*"
An IP address and port could be used instead of the FQDN. Further specific hosts can be specified using the host name instead of the * wild card.
To remove the Syslog entry the following command would be used.
Set-VMHostSysLogServer -SysLogServer $null -VMHost "*"