How to cancel iTunes

No! iTunes is a store for digital media and also a jukebox and device management app for Mac and Windows. On the Mac it was split in 2 since Catalina, some features went to Finder and some stayed with the now called Music app. For Windows it is still iTunes.

To cancel additional iCloud storage see Downgrade or cancel your iCloud storage plan - Apple Support

Rafael provided the right link for you to cancel additional iCloud Storage. But before you do, are you sure you don't need it? If you have more than 5GB of data in the cloud, without additional iCloud Storage, you will no longer be able to backup to iCloud.

iTunes is a store, not a subscription. What is it you purchased you want to cancel?