ESXi embedded is usually an OEM version of some vendor installed on a SD or flash memory. It cannot be download from VMware site.
ESXi installable can be obtainted from VMware site and can be installed both on hard disk or SD/Flash (see also: )
But both are ESXi, so you can use VUM or vSphere Host Update (with vSphere client) to update your system.
Note that there are also some vendor customized versions of ESXi installable that include other software (like hardware monitor or similar) that usually are included in the embedded version.
Andre
Andrew | http://about.me/amauro | http://vinfrastructure.it/ | @Andrea_Mauro