[j-nsp] How to upgrade junos 5.0.0r8.1

[George]

Hi there,

I have two juniper netscreens one is Firmware 5.0.0r8.1 . Now I have
encountered a problem when setting up a VPN on this one due to firmware
version thus I need to upgrade it.

The question is how do I upgrade this firmware, challenge being that it
is running live services and if the upgrade fails how do I roll-back.
Guess the thing is I have to be 100% sure the upgrade will not affect
anything.

Cheers.
George