Before flashing new firmware, you should boot into recovery. After a failed upgrade, Lineage typically boots into recovery automatically. If you've already reboot back into the OS, below are the instructions to get back there:
- Plug your phone into a PC and open a terminal, run:
adb devices
You should see a device ID appear, with List of devices attached.
- Run:
adb reboot bootloader
You will then boot into the bootloader. Use your volume key to search for Recovery, hit the power key once you're on it.
You should then boot into recovery where you can continue flashing the new firmware.