Cesar,
ISXC 5.0.x does not have any SNMP mass configuration tools, but ISXC 5.1.x and above do have the SNMP Configuration tool. This will allow you to create a template that includes the updated credentials and allow you to roll this out to all of the other SNMP devices.
If you have valid software support contracts, call our US tech support team @ 877-908-2688 so that we can verify your support contracts and release the software upgrades to you.
Alternatively, you can use this process that bypasses the ISX Central:
How can I mass configure an Network Management Card (NMC) or NMC embedded product?
Using this process, you can create a custom config.ini that includes only the updated username/password, and create an upload list of all the devices you want to upload that config change to.
Hope this helps!
-Mike