We don't store any card details on our website. This is to maintain the highest level of security possible. You payment details are handled by a third party called SagePay, one of the largest and most trusted payment processors in Europe.
The downside to this is that you can't simply enter a new card number or expiry date for future payments - you need to enter your full card details and billing address.
If you want to upgrade from monthly plan to a one year plan, you can do this at any time and any remaining time on your monthly plan will be added on to the end of your new plan.
If you want to continue on a monthly plan, but change your card details, the easiest way to do this is allow your account to expire and then log in again. You'll be automatically sent to the payment page to choose a new plan and enter your new card details. You won't lose anything by doing this.