Yes it is a free preview right now (18 Aug 2015). Just install the new Office 2016 Trial from here. Here is the list of new features. If you have the appropriate version of Office 365 you will receive the upgrade automatically when the product is released.
Test it using a separate PC or a virtual machine. DO NOT install it on your regular work machine because it is still in testing stage.
I will cover the features in more detail. But here is a very interesting feature which some of you may like. You can do forecasting very quickly using the Forecast Sheet feature of Excel 2016.
Here is the raw data – a time series and the forecast generated from it.
More coverage of new features coming soon…
(I had written another post about this trial, which I have deleted. That was because the side-by-side running of new version with older versions mentioned in that article requires lot of technical explanation, which is not worth the trouble at trial stage)