You can cancel where you originally signed up:

If you signed up on our website:

You can unsubscribe anytime by logging in to your account, selecting the user icon on the top right, and going to "Settings" > "My Subscription".  Find the tier you’re subscribed to and click the “Unsubscribe” button below it.

If you used PayPal: You can cancel recurring billing in your PayPal subscriptions

If you registered using ApplePay, then unsubscribe by going to your iCloud settings by clicking your name in the settings app, and then selecting "Subscriptions".  Find S&S under the active subscriptions list and click "Cancel Subscription".

If you registered via Google Pay: You can unsubscribe by logging into your Google Play manager and canceling your subscription there

You will continue to be able to access +PLUS content until your billing period expires.