If by premium you mean x10premium.com, then you can cancel it by logging into https://clients.x10hosting.com, finding the appropriate service, and hitting the Request Cancellation button at the bottom
If by premium you mean Prime or Illuminated, you can cancel the subscription by logging into paypal and cancelling it
When you request cancellation, the service will continue for as long as you've pre-paid (if you pay on a per-month basis, it will last until the end of the month, per-year at the end of the year, etc.); at least, that's how it worked for me back when I cancelled my premium account. I'm not entirely sure if it still works this way so I'd recommend opening a ticket at https://clients.x10hosting.com as stpvoice recommended.