Deleting an existing API key is recommended to be a very infrequent action. ALL the multiple applications that interacted with this API to transfer data to each other will be terminated with the deletion of this API key. There is no way to recover an API once it's deleted, only way is to recreate one and program it for inter-application communication.
Once you are absolutely sure you want to delete the API key, click the 'DELETE' button on your site:
The DELETE button will give you a warning message specifying the consequences of the action.
If you are sure to delete the API, click on ‘delete API key’. In the above screenshot, the already made API is been asked to delete.
When you are ready to continue, press 'DELETE SITE'. Once the API key is deleted, you will be left with no API key and you will be needed to create a new one for seamless site performance.
NOTE: Deleting API key does not cancel your plan. Your plan is still in effect and you could create an entirely new API for your existing site as you did previously.