Hi there

HDR should activate itself automatically upon detection of the HDR metadata once playing suitable HDR videos.  Some test files can be located HERE, or if you have the 4K Netflix package you can test (even sign up to Netflix for 1 month free, if you have not used Netflix for a while).  As well as there are some HDR files on YouTube.

Once playing a HDR video, goto the "Action Menu" and you should notice HDR in the "Picture Mode" area.

Cheers