Encountered error 6 when upgrading Iphone 5s from 7.1.2 to 8.3

Question

I encountered error 6  when upgrading Iphone 5s from 7.1.2 to 8.3
I've tried upgrade iphone on three computers , the results are all the same.
One of the computers successfully upgrade my ipad to 8.3.
There are 2GB left on IPhone 5S before I perform the upgrade.
It should be enough for iTunes to finish the process, right?
But in the log, it says  "copier_file_error: /mnt2/mobile/Library/PreinstalledAssets/com_apple_MobileAsset_Font.cpio: No space left on device"
why is that?
Any way for me to continue the upgrade?  it seems it's almost done.
Thank you
the end of the log  is as below:
[10:36:29.0714] Could not find libAppleSWDHIDFlashUpdater.dylib
[10:36:29.0714] entering fixup_var
[10:36:29.0714] entering update_var_directory_hierarchy
[10:36:29.0714] replace_data_partition_path: moving /mnt1/private/var/mobile/Library/PreinstalledAssets to /mnt2/mobile/Library/PreinstalledAssets
[10:36:29.0714] copier_file_error: /mnt2/mobile/Library/PreinstalledAssets/com_apple_MobileAsset_Font.cpio: No space left on device
[10:36:29.0714] copy /mnt1/private/var/mobile/Library/PreinstalledAssets to /mnt2/mobile/Library/PreinstalledAssets failed: 0x2
[10:36:29.0714] unable to copy directory: 2
[10:36:29.0714] failed to replace path mobile/Library/PreinstalledAssets: 2
[10:36:29.0714]
[10:36:29.0714] ==== end of device restore output ====
[10:36:30.0280] AMRAuthInstallDeletePersonalizedBundle
[10:36:30.0421] <Restore Device <<<<<IMEI>>>>>>0>: Restore failed (result = 6)
[10:36:30.0542] Restore completed, status:6
[10:36:30.0542] Failure Description:
[10:36:30.0542] Depth:0 Error:AMRestorePerformRestoreModeRestoreWithError failed with error: 6
[10:36:30.0542]    Depth:1 Error:無法完成作業。（AMRestoreErrorDomain 錯誤 6 - Failed to handle message type StatusMsg）
[10:36:30.0542]       Depth:2 Error:無法完成作業。（AMRestoreErrorDomain 錯誤 6 - failed to fixup var）

Answer 1

Hello there, Kenduck.
It sounds like you are trying to update your iPhone 5s to the most current version of iOS and are getting an Error 6 message. The following Knowledge Base article offers up some great information in regards to that very error message:
Get help with iOS update and restore errors - Apple Support
Particularly:
Check your security software
Errors: 2, 4, 6, 9, 1611, 9006.
Sometimes security software can stop your device from communicating with either the Apple update server or with your device.
Check your security software and settings to make sure that they aren't blocking a connection to the Apple servers.
If you still see the error message, check for USB connection issues.
Thanks for reaching out to Apple Support Communities.
Kind Regards,
Pedro.