- Go to the Upgrade page.
- Enter your usual login details and click Log in.
- Check I agree to the Terms of Use and click Upgrade my account.
Your Premium will start immediately, but you won't be charged yet. Instead, your remaining Unlimited time will be halved and converted to Premium.
If this doesn't work, you can contact support for assistance in getting your unlimited upgraded to premium: https://www.spotify.com/about-us/contact/contact-spotify-support/