first of all, as i wrotte above, multiboot in this receiver, isn't plugin, and all images are in the flash.so does not matter which image is in slot1 /2/3/4 ...
if you still want to change the raw for any reason...
download the openpli image that you want. not the multiboot one, the "full"
unzip and place the folder in a fat32 formated usb stick
unplug the receiver from power supply an put the usb disk
plug the power supply and in the first screen message, press "ok" .
wait to finalize, and the receiver will reboot.
openpli will be in 1st slot of your receiver.
and a video how to