Just as a quick reminder to myself. You can actually get away with overwriting OpenWRT the quick and dirty way on a device like my ALIX 2D3, which can only boot from CF card, without having a card reader.
The only prerequisite is that you have a previous OpenWRT version running on the device.
After downloading the new image file to a client machine that is connected via Ethernet to the OpenWRT device in question, prepare a Netcat instance like this:
$ nc -l -c -p 50001 < openwrt-21.02.0-rc4-x86-geode-generic-squashfs-combined.img
On the OpenWRT box, for which I would recommend to login via serial connection, the image can be obtained and flashed right away using
$ nc 192.168.0.11 50001 | dd of=/dev/sda bs=1M
Keep in mind that overwriting the file system of a running OS is potentially dangerous and can lead to data loss or file system corruption! This is quick and dirty!