Assuming you are asking about the Network Analysis Module for Catalyst 6500, you need to have a support contract. With that, you can download the new software:
Downloads Home > Products > Switches > LAN Switches - Core > Cisco Catalyst 6500 Series Switches > Cisco Catalyst 6509-E Switch > Cisco Catalyst 6500 Series Network Analysis Module (NAM-1/NAM-2)
(ISR or branch router NAM software is similarly available under those respective products' download pages.)
Upgrades are possible up through Version 4.x. Version 5.x or later requires a fresh install if you are on a pre-5.x release and also has certain memory requirements as a prerequisite. All the procedures are shown in detail in the Release Notes for the version you are moving to.
