rewrite linux image on device via ethernet

I have embedded device with Linux (stored in EMMC). When i need to update Linux image (i have one .img file) i need to connect via serial terminal, edit boot_targets to start system from SD-card and then rewrite EMMC from host via dd command. But what if i have device in another city and i can only use remote SSH to connect to it. How can I update new Linux image then ? I tried to make copy of existing fs to tmpfs, but get stuck at porting existing fs to tmpfs...