How do I change the frequency of heart rate readings

I never had the Series 2. I had the original, then a 3, and now have a 4. I don't ever recall seeing a place to adjust that. If you believe that was an option, then you may wish to use the Contact Support lin at the top of the page here to speak to Apple and ask them.

I don't believe this is something that can be user adjusted. I believe the default is around 5 minutes, unless you are exercising and in an activity when it is more often. Many of the activities it is constant.