I am sorry to hear that you don’t succeed in updating your FP OS. Your OS is a changing system (with every update/new app etc. you are changing it), it is not static. That’s why problems may occur. You could try wiping cache and data of the Updater app (to do so, go to Settings > Apps > All Apps > Updater, wipe cache and data) and try to download the update again. Please also have a look into this useful wiki for troubleshooting.