This article provides with the steps to upgrade the "upgrade-failed member" in a 2 member Virtual Chassis.
If we are upgrading a 2 unit VC and after the upgrade one of the member is upgraded while other member is on the old version.
We can upgrade the failed member with the help of a USB device.

Upgrade of one of the members in a 2 member Virtual Chassis fails to upgrade to the desired code version.

For example we have FPC0 and FPC1 as 2 members in a VC with FPC0 as master.

1. When we are in a state where FPC0 is upgraded while FPC1 is on the old version the FPC1 will be in "Inactive" or "Not-Prsnt" state.

{master:0}
root@QFX5100> show virtual-chassis    

Virtual Chassis ID: ce3d.a300.a929
Virtual Chassis Mode: Enabled
                                                Mstr           Mixed Route Neighbor List
Member ID  Status   Serial No    Model          prio  Role      Mode  Mode ID  Interface
0 (FPC 0)  Prsnt    VB3715300239 qfx5100-96s-8q 255   Master*      N  VC
1 (FPC 1)  NotPrsnt VB3715280106 qfx5100-96s-8q


2. Check from the console if the "Upgrade Image Jinstall file" is present in the /var/tmp directory of the failed member.
3. If the "Upgrade Image Jinstall file" is present then proceed with the image installation from the console of the failed unit.

>request system software add /var/tmp/ <Image file>
>request system reboot.

4. If we do not have the "Upgrade Image Jinstall file" on the failed FPC then we have to transfer that file to the failed member using the USB. 
5. We should have the in-band access to the device via FPC0 as  member 0 is still active.
6.  FTP/SFTP the "Upgrade Image Jinstall file" to the FPC0 (master) in the /var/tmp directory.
4. Now we can insert the USB into the FPC0, mount the USB and then transfer the "Upgrade Image Jinstall file" to the USB. Please use the below KB to mount USB and transfer files to the USB.

http://supportportal.juniper.net/s/article/Junos-How-to-mount-a-USB-drive-on-EX-SRX-MX-QFX-Series-platforms-to-import-export-files?language=en_US


5. Insert this USB into the FPC1 (Upgrade-failed member) and then mount and transfer the Upgrade Image file to member 1 /var/tmp folder.
6. Perform the md5 checksum on the Image file.
6. From the Console of member 1 upgrade the device using the following commands 

>request system software add /var/tmp/<Image file> 
>request system reboot