You can upgrade from 6.0 to 6.3 directly. But the Juniper signing keys have changed and you will also need to update the certificate as the first step in the upgrade process
1-upload new signing key
2-upgrade ScreenOS
If the option to update the signing key is not in your version of ScreenOS you will need to open a support case to get an intermediate version to help with this.
Signing Key Articles
http://kb.juniper.net/InfoCenter/index?page=content&id=TSB16495
http://kb.juniper.net/InfoCenter/index?page=content&id=TSB16496
Also if this is a SSG branch device, be sure to turn off DHCP server on any internal intefaces as a precaution. There are some circumstances where the new OS will get into a crash boot loop when this is enable right after an upgrade. You can turn it back on again afterwards. This is on in the default config for bgroup0.
Save current config
turn off DHCP server
Upgrade
Reload current config