refer the following articles. It will help you in creating bootable USB of ESXI 5.1. Its same as the bootable ISO CD/DVD.
Boot with the USB and follow the steps in installer for upgrading the host.
Create bootable USB key with ESXi 5.1 | VMwaremine - Mine of knowledge about virtualization
Creating (a better) vSphere 5 ESXi embedded USB Stick (HP) | VMwire