Hello,
You can use the link below for the upgrade guide:
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_upgrade.pdf
Using link below you will be able to download 6.3.0r3 provided you have an account:
https://www.juniper.net/support/downloads/?p=isg1000#docs
For 5.4.0r8, you may need to open a TAC case.
Regards,
Rushi