Documentation on the FunnelFlux APIs can be found directly from your tracker via the knowledgebase menu, i.e.

Note: accessing from this area will load the documents from within your FunnelFlux installation, which allows direct execution and testing of your tracker's API.

You can separately view these documents below:

API Documentation for Statistics:
https://api.funnelflux.com/stats/

API Documentation for Data:
https://api.funnelflux.com/data/

Note: The new API has a FunnelFlux version requirement of 1.6+. If your API request is still using the old API, then it will still work with FunnelFlux version 1.6+