Here is all written up:
Basically it depends what version of EdgeOS you had before you installed OpenWrt (because of the bootloader).
You can try if the Ubiquiti recovery (it will just do nothing if you have the old bootloader):
If it is not working you have to use either serial console (usb-ttl-adapter) or follow:
"Back to EdgeOS without TFTP/serial console access on devices with old Bootloader" of they guide linked 1st in this post.
But be warned this method can fail and you have to build your recovery file by your own because it is not open source. If it is failing you have to use a usb-ttl-adapter method.