If you go to the upgrade section, does it let you select premium? If you can't change it that way, the best option will be to get in touch with Spotify support using this Support Form or on Twitter @SpotifyCares. If you use the support form, please be sure to reply to any automated messages you get, even if from a no-reply address, to get in touch with someone who can help you more quickly. Thanks!
dinomightRock Star 11
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