Hey there and welcome to the community @djina-021,
When you signed up to Spotify Premium, did you pay via iTunes? iTunes is priced differently.
If you did make a payment via iTunes, you will need to cancel the subscription with iTunes and subscribe to Spotify directly on the website when your current billing period has expired. Once your subscription has reverted to Free, you will then be able to re-subscribe to Premium on the Spotify website here.
You can find out how to cancel the iTunes subscription here. You can also request a cancellation and refund for the payment via iTunes or by contacting Apple directly.