Hey! Welcome to the community :)
NUS is great isn't it! :D
Just go to https://www.spotify.com/uk/nus/ and then press the massive "Sign up" button, it will take you to a page where you can select "Already have a Spotify account? Click here to log in", then just sign in using your premium account details, enter your NUS card number and Date of Birth and your discounted subscription will be applied from your next payment date.
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