Unable to Update HTC One M8 to Android 6
SOLVED: OK I couldn't get this update either tried moving as much as I could to the SD card had over 16GB free still wouldn't work till I wiped the cache.

Here are the steps to wipe the cache
1) Hold power button
2) Press restart
3) As soon as you see the HTC logo, hold the down volume button
4) If done correctly you should be met with a white screen, from there go down to recovery and press power button
5) Within a minute, you should be greeted with black Android System Recovery Page
6) From there click Wipe Cache Partition
7) After that is done, go to reboot system now and once in android again, download update.

You must have battery at least 35% charged to install the update.