From you description I assume you have run
requests system storage cleanup
Also look at the home directories that may have files and /var/tmp for old software install files
If you can't get enough space that way, don't copy the file to the device but put it on a usb.
mount the usb to the switch
https://kb.juniper.net/InfoCenter/index?page=content&id=KB12880
do not copy the file but run the install from the mounted usb directory instead
request system software add /var/tmp/usb/filename