To upgrade to v6 of our APIs, go to your Developer Console, and create an app to get an API key for v6 endpoints.
You can still use v4/v5 endpoints after you get your v6 API key. You can transition to v6 on an endpoint-by-endpoint basis, or you can begin using v6 endpoints exclusively at any point.
Please note, however, that your v6 API key cannot be used to send requests to v4/v5 endpoints, and vice versa. Also, there are differences between the versions' requests and responses; attempts to upgrade by replacing "v5" with "v6" in your requests will cause your integration to fail.