Here’s an easy three step process to update your ESXi host to the latest build using a USB Flash Drive.
Step1- Download latest ISO VMware vSphere Hypervisor(ESXi) 6.5 ISO
Step2- Download and run Rufus
Step3- Click the CD icon and point to the ISO you downloaded, leave everything else default and click start
(Note you may get a popup requesting to replace the menu, to avoid issues with the USB key not booting you’ll need to click Yes to download the updated file.)
I have performed this method on versions 5.5 and 6