if you just got your unit, it's not adviced you install a BETA OS; better wait for the official release.
Anyway after downloading Eclipse V3.5 Beta 4 Update – Compact Flash, unzip it and copy the files to the card using an usb Compact Flash Card reader.
The files must be copied to the card, NOT in a folder.
Then insert the card and power Eclipse on pressing and holding the 1 KEY.
The unit will walk you thru the update.