Check is this ISSU feature is supported: https://apps.juniper.net/feature-explorer/issu.html
Upgrade routers with dual Routing Engines:
https://www.juniper.net/documentation/en_US/junos/topics/task/installation/junos-software-installing-on-dual-routing-engines.html
...
Step-by-step example for ISSU:
https://www.juniper.net/documentation/en_US/junos/topics/task/installation/issu-upgrading.html
ISSU for QFX5100 ( not in virtual chassis )
https://nextheader.net/2015/09/24/in-service-software-upgradeissu-on-qfx5100/