It seems that your battery is deep discharged. Connect it with a charger and wait 30 mins.
If the LED is still flashing red, your battery is dead or your phone can’t charge it due to a damaged boot and/or system partition.
Turning on the phone with a connected charger results in reboots due to the drained battery.
Flashing the phone without inserted battery might help:
Follow the steps described in Installing Fairphone Open OS using Fastboot (Step-by-step Guide)
Start your phone into fastboot mode as follows:
- Remove the battery.
- Press and hold volume down and power button, then connect the phone to your pc. The fairphone boot logo appears. Now, release both buttons.
Your phone is now in fastboot mode. If the phone still reboots, verify if both buttons were pressed while connecting the phone.
Install the previous 17.04.0 version as described in the wiki above.
Insert the battery and connect the phone to a charger (don’t turn it on). The LED will start flashing red, then flashing should stop and the battery loading animation shows up.
Your data should still be in place since the manual update package does not contain a data partition, and the flash.bat and flash.sh scripts do not contain wipe commands.
You may now try again the update to Android 6.