Hi .. as per the below link upgrading from 4.1 to 5.1 is two steps procedure.
1.- Upgrading from 4.1 to 5.0
2.- Upgrading from 5.0 to 5.1
http://www.cisco.com/en/US/products/hw/vpndevc/ps4077/products_installation_guide_chapter09186a008055fc78.html
For upgrading from 4.1 to 5.0 youc an download the software to a local ftp server and follow the below steps.
Log in to the CLI using an account with administrator privileges.
Step 3 Enter configuration mode:
sensor# configure terminal
Step 4 Upgrade the sensor:
sensor# configure terminal
sensor(config)# upgrade ftp://FTPusername@ftp_serverIP_IP_ddress//directory path/IPS package name-pkg
Step 5 Enter the password when prompted:
Enter password: ********
Re-enter password: ********
Step 6 Type yes to complete the upgrade.
Note Major updates, minor updates, and service packs may force a restart of the IPS processes or even
force a reboot of the sensor to complete installation.
Step 7 Verify your new sensor version:
sensor# show version
Application Partition:
Cisco Intrusion Prevention System, Version 6.0(1)S149.0
NOTE: as of version5.X you need a license for getting signatures updates.
You can repeat similar procedure for upgrading from 5.0 to 5.1
I hope it helps .. please rate it if it does !!!!
