Even though I agree with @lleachii, you should be able to find some useful info here Tips for getting cheap used x86-based firewall with full Gbit NAT (a PC Engines APU) if you are in the US
When I updated mine (D1), I booted a specific image (not openwrt) for flashing.
Probably the TinyCore from https://pcengines.ch/howto.htm#TinyCoreLinux