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
These are for ISSU:
-Verify unified ISSU support for your device
-Perform a unified ISSU
-Verify that the unified ISSU is successful
-Understand how the unified ISSU process works
https://www.juniper.net/documentation/en_US/junos/topics/concept/issu-oveview.html
Step-by-step example for ISSU:
https://www.juniper.net/documentation/en_US/junos/topics/task/installation/issu-upgrading.html