Open the App Store on your iPhone
Search for Spotify
Install the Spotify app if it is not already installed
Open Spotify and sign in to your account
Tap Premium or Go Premium
Choose a Premium plan
Confirm the purchase with your Apple ID, Face ID, or Touch ID
Complete the subscription payment
If you already have Premium, sign in with the same Spotify account on your iPhone
If you subscribed elsewhere, make sure the subscription is active on the correct account