Upgrading VMware vSphere environment.
Prepare your environment before upgrading.
Requirements:- 1)first upgrade the vCenter server.
2) check host hardware compatibles to support esxi versions.
3) Update host firmware version
(For Ex. HP server? update system firmware ILO and BIOS, controller version)
(If cisco UCS? check the ucs manager and FI versions 3.0 or 3.2, 4.0 version compatibility)

There are 3 ways to upgrade the ESXi version.
1) Download customized ISO from VMware. And Enter Maintenance mode, connect physical console ILO or iDrac and install it.
vim-cmd /hostsvc/maintenance_mode_enter , exit
2) From vcenter server through update manager, create a base line and enter maintenance mode and attach & scan and remediate it.
3)via CLI – Two methods , one is offline and online.

In case you having problems after upgrading of ESXi version, it can revert back an upgrade ? Yes, you can restart your ESXi server and simply hold
SHIFT + R and chose to roll back.

Hope this will be informative for you, Thank you for Reading !