You can renew your parking permit online if your address and vehicle details are the same as they were when your last permit was issued. It can take up to 10 days for your permit to arrive. A grace period of 10 days will be allowed if your new permit doesn't arrive before your current permit expires.
You can only renew a permit up to 20 days before or 10 days after the expiry date. If it is more than 10 days since your permit expired you will need to apply for a new permit.