What needs to be considered before upgrading vSphere ESXi.

Products

• VMware vSphere ESXi

Issue/Introduction

There are many things to be aware of when performing an upgrade of ESXi hosts. Various resources are available to help users be familiar with how to best prepare ahead of time to best ensure the likelihood of a smooth and successful upgrade process. 

Resolution

If you're upgrading to ESXi 8, a good starting point is Install or upgrade ESXi from 7.x to 8.x. This is a KB article written specifically for guidance in installing or upgrading vSphere ESXi. It provides a numbered list with each step in those processes and links to the documentation for how to do those steps.

If you prefer to just start with the official instructions for vSphere users and administrators, Broadcom's Techdocs, here's a good place to start and get familiar with everything to check on ahead of an upgrade and what to expect during and after the ESXi upgrade process:
 

• Before Upgrading ESXi Hosts - This page is primarily a short list of best practices and links for the different available upgrade methods. 
• Overview of the ESXi Host Upgrade Process This page illustrates the overall upgrade process and provides links for each of the specific areas to check and links to the instructions for the kind of upgrade being done (through GUI, scripted, esxcli commands, etc.).

From either of the links above, there is a navigation pane on the left of the page with many more pages specific to Requirements and Considerations before an upgrade such as ensuring hardware compatibility, that necessary free disk space is available, and so forth - like the examples in the image below: