Juniper JUNOS OS 10.4 - RELEASE NOTES REV 5 Release Note page 79

Upgrade and Downgrade Instructions for Junos OS Release 10.4 for M Series, MX Series, and T Series Routers
NOTE: Before upgrading, back up the file system and the currently active
Junos configuration so that you can recover to a known, stable environment
in case the upgrade is unsuccessful. Issue the following command:
user@host> request system snapshot
The installation process rebuilds the file system and completely reinstalls
the Junos OS. Configuration information from the previous software
installation is retained, but the contents of log files might be erased. Stored
files on the routing platform, such as configuration templates and shell scripts
(the only exceptions are the juniper.conf and ssh files) might be removed. To
preserve the stored files, copy them to another system before upgrading or
downgrading the routing platform. For more information, see the Junos OS
System Basics Configuration Guide.
Copyright © 2011, Juniper Networks, Inc. 79