In your browser, you have to copy this link : www.flymeos.com/firmware.html.
Click on your phone (M8).
Choose to downgrade the OS : Flyme 7.1.5.1G (if you had 7.1.6.0G). Download the version 7.1.5.1.G
When you are finish, put your phone in recovery mode (Volume Up + Power). When you see the flyme logo, release the buttons.
Connect your phone to the computer. You have have a recovery disk in explorer. Copy your download file (update.zip) in recovery disk. If you don't have it the first time, try again!
After that, check Upgrade and wipe data and press start after.