Should happen automatically.
But since you can access Luci, we’ll use that😄
Go to System/Backup / Flash Firmware & select Flash Firmware.
Select the latest V3 image. If it says that verification check fails, select the force option. Don’t keep settings.
After this is done, you’ll boot into uboot, but Luci etc is completely removed. Now you should be able to flash the latest V3 image again in Uboot.