Is kind of hidden but is there.
go at this link first which says:
I quote
Upgrading or Downgrading the Software
To upgrade from 2.x or 3.x to 4.0, see the "Managing Software, Licenses, and Configurations" chapter in the Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Configuration Guide using the CLI. Be sure to save a copy of your 2.x or 3.x configuration if you later want to downgrade.
http://www.cisco.com/en/US/docs/security/fwsm/fwsm40/release/notes/fwsmrn40.html#wp161551
then go here
Managing Software, Licenses, and Configurations
http://www.cisco.com/en/US/docs/security/fwsm/fwsm40/configuration/guide/swcnfg_f.html#wp1049763
Regards
Jorge Rodriguez
