GrabzIt uses PayPal for payments. If you paid with your PayPal account you can update the payment information by logging into your PayPal account.
Otherwise if you used a credit or debit card without a PayPal account you will need to upgrade again using your new payment details. You will receive a number of free days to cover the amount you have already paid for. The old subscription you had with GrabzIt will then be automatically cancelled.
Your Application Key and Application Secret will not be effected by your old subscription being cancelled, and will remain the same.