If your free messages start date is 2017-12-05, unless you have used up 3 million messages in two days (very very unlikely), you should be able to update your channel.
Can you log out of ThingSpeak, log back in and confirm that the message about having run out of messages is gone? Can you try updating a test channel with a single GET using your browser - does that update your channel?
If the answer to either of the above two questions is 'no', please send me an email vcherian(.at)mathworks (.com) and we can set something up to investigate this.
