You can easily update your vCenter Server Appliance (VCSA) offline by downloading a product patch ISO and via the CLI.
Download the latest patch from VMware Patch Download Center. Select VC from the Search by Product drop-down menu, then select the correct version (i.e. 6.0 or 6.5). Download the patch ISO and attach it to the VCSA.
Log in to the shell (using SSH or VMRC) and initiate the update:
software-packages stage --iso
software-packages list --staged
software-packages install --staged
After the update has finished, you will have to reboot the VCSA.
You can use either esxcli or vicfg-hostops to enter/exit maintenance mode.
Note that you can use vicfg-hostops --operation info to check whether the host is finished with entering maintenance mode or is still busy.
After connecting to the host or vCenter server:
You can easily test the availability of an NTP server by using ntpdate. This utility is a command line utility which performs a one-time time sync to the specified NTP server but it can also be used to query the time without setting it by using the -q parameter:
$ ntpdate -q 10.30.0.1 server 10.30.0.1, stratum 4, offset 0.001217, delay 0.04170 8 Jan 10:19:36 ntpdate: adjust time server 10.30.0.1 offset 0.001217 sec