You'll be prompted to update your credit card if the credit card expiration date you entered is approaching. Our system will attempt to keep a current credit card on file, so even if your current subscription is not due for renewal, you may be prompted to give us your new card details.
You can cancel automatic renewal under your account's billing page if you like, which should also end the messages you've been getting.
If you have further questions about turning off renewal, please see our article "How do I turn off auto-renewal?" to cancel future billing.