Once you've downloaded the ESXi 6.7 Update 3 ISO file, you can install it on your host machine. You can use tools like Rufus (for Windows) or Etcher (for macOS or Linux) to create a bootable USB drive.
Differences between vSphere 6.7 and 7.0 and how to upgrade - ITECS download vmware vsphere hypervisor esxi 67 2021
You have two options. Choose wisely based on your hardware: Once you've downloaded the ESXi 6