How to upgrade the firmware without the paid products
The primary way
to manage an ESXi system is via Windows. To this end, you will need the
VMware vSphere CLI package (available from the VMware website)
installed as well as the vSphere GUI client. Both are available for
Windows; however, only the first is also available for Linux. The GUI
client will not allow you to upgrade the firmware of ESXi, which is why
you need the CLI tools (but you do need the GUI client to
manage/shutdown/suspend the virtual machines in order to do the
upgrade). So, because the client only runs on Windows, you will either
need a Windows system available, or a virtualized Windows install on
another machine that is separate from the ESXi host.