How do I cancel an iTunes subscription?

If you mean Apple Music, click here, follow the instructions, and wait for it to expire. If you need to cancel it immediately, contact Apple.

iTunes Music is based on a per-item payment model and doesn't use subscriptions.

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iTunes itself does not offer any subscription.

If you mean Apple Music:

Manage your Apple Music membership on your iPhone, iPad, iPod touch, Mac, or PC - Apple Support

For Subscriptions to other Apps using your Apple ID:

View, change, or cancel your subscriptions - Apple Support