Navigate to juniper website and download junos 12.3R3.4 release package.

Open your web browser and log in to the juniper switch

Take a look of the current JUNOS release, it is 11.4R7.5

Go to -> Maintain -> Software -> Upload Package and choose the downloaded package.

We have to wait a few minutes, after that we can check the current version of JunOS

The upgrade process to JunOS 12.3R3.4 was successful 🙂