To determine possible options please answer the following questions if possible:
- Is this a stand alone ESXi host running with the free Hypervisor license, i.e. not connected to vCenter Server?
- Was the host initially installed with ESXi 4.1, or has it been upgraded from a previous version (e.g. ESXi 3.5)?
- Is ESXi installed on an SD/USB device or on the HDD?
- What hardware do you use? If the hardware is supported for ESXi 5.x, re-installing the host (preserving the VMs) might be an option in this case.
In any case ensure that you have an up-to-date backup of the VM's!
André