You'll need to register your server's serial number at ESS portal:
https://www.ibm.com/servers/eserver/ess
Once there, sign in there with a valid IBMid account (AFAIK, you can create one account for free), go to 'My entitled software' area, then 'Register customer number', and look for Authorization section. Locate 'Hardware or Software serial number' textbox, input your server's serial number, and click on 'Submit' button. After that, click on 'OK' and you'll see a new area called 'Software downloads', where you can download all SW packages assigned to that machine.
Regards.