Software download link: https://education.ti.com/en/software/search/ti-nspire-cx-cas Select the Student software that corresponds to your license number
Extended video walkthrough on how to download software and activate your license:https://youtu.be/6YYucuq3Zaw
Link to the nspire beginner start guide made by Texas Instruments.
Link to the operating system that ndless supports:
For CX II CAS, use this: https://education.ti.com/download/en/ed-tech/8CBC7A5F94D447B39E7C8C7F8178DB28/01A6CF84FDE34A9DB36088D3D931A410/TI-NspireCXIICAS-5.3.2.129.tcc2
For CS II, use this: https://education.ti.com/download/en/ed-tech/2726D6FC251C40648FDEE5AFABED3B95/92FF1CCDE36C49FD94BCB92420B70960/TI-NspireCXII-5.2.0.771.tco2