Following error is seen while upgrading the QFX5100 device:

"THIS IS A SIGNED PACKAGE

ERROR: /usr/libexec/ui/downgrade: a legacy Junos installation package is required..."

The upgrade fails and shows the above error while pushing the file.

The above-mentioned error can be seen during the upgrade if the current device is running QFX-5e image, and the target version is a QFX-5 image.  

That E stands for Secure Boot and when trying to go with a one without secure boot.

To upgrade from 5e to 5 image you should perform a recovery installation, following the link below: 

https://www.juniper.net/documentation/en_US/junos/topics/task/installation/junos-software-reinstallation-preparing-qfx5100.html

or Format install, 

https://kb.juniper.net/InfoCenter/index?page=content&id=KB34749&cat=QFX5100&actp=LIST&showDraft=false [juniper.net]

However, you can do a normal upgrade on the device using a QFX-5e image for the target version.