To determine the version and update your ESXi hosts are running, use the following method. It is assumed that vCenter is running in your environment.

 

First of all we need to get build number:

Log in to the vCenter using the web client
Click Hosts and Clusters -> {your vCenter} -> {your Datacenter} -> {your cluster} -> {specific host}.
Under Summary -> Configuration you'll see the ESX/ESXi version. 

 

 

When we have obtained the build number we need to cross-reference this using VMware KB2143832

A snippet is taken below:

Version	Release Date	Build Number	Installer Build Number
ESXi 6.5.0d	2017-04-18	5310538	NA
ESXi 6.5. Express Patch 1a	2017-03-28	5224529	NA
ESXi 6.5. Patch 01	2017-03-09	5146846	5146843
ESXi 6.5.0a	2017-02-02	4887370	N/A
ESXi 6.5 GA	2016-11-15	4564106	N/A
ESXi 6.0 Patch 5	2017-06-06	5572656	N/A
ESXi 6.0 Express Patch 7c	2017-03-28	5251623	N/A
ESXi 6.0 Express Patch 7a	2017-03-28	5224934	N/A
ESXi 6.0 Update 3	2017-02-24	5050593	N/A
ESXi 6.0 Patch 4	2016-11-22	4600944	N/A
ESXi 6.0 Express Patch 7	2016-10-17	4510822	N/A
ESXi 6.0 Patch 3	2016-08-04	4192238	N/A
ESXi 6.0 Express Patch 6	2016-05-12	3825889	N/A
ESXi 6.0 Update 2	2016-03-16	3620759	N/A
ESXi 6.0 Express Patch 5	2016-02-23	3568940	N/A
ESXi 6.0 Update 1b	2016-01-07	3380124	N/A
ESXi 6.0 Express Patch 4	2015-11-25	3247720	N/A
ESXi 6.0 U1a (Express Patch 3)	2015-10-06	3073146	N/A
ESXi 6.0 U1	2015-09-10	3029758	N/A
ESXi 6.0.0b	2015-07-07	2809209	N/A
ESXi 6.0 Express Patch 2	2015-05-14	2715440	N/A
ESXi 6.0 Express Patch 1	2015-04-09	2615704	2615979
ESXi 6.0 GA	2015-03-12	2494585	N/A

 

As you see highlighted above we can see that this particular host is running ESXi 6.0 update 2