I have two versions of my website- www and non www. The non www ranks higher than www. Will redirecting the high rank(i.e non www) to low rank (i.e. www) page reduce the overall page rank or the rank will be the average of both pages?
Technically, if you redirect to the www version, all equity will be passed from the non www version. However, if you are using tools to view 'page rank' or 'page authority', this may not necessarily update due to signals that they look at (e.g. links to that exact URL as opposed to links to redirected URLs).
If you want to retain the higher rank but still redirect then you can use the rel canonical tag to achieve the same.
Just keep the canonical tag pointing to the non www version
You can set your preferred domain in your Google Search Console (formerly webmaster tools. This tells Google which version (www or non-www) you prefer, and combines authority you've built up with both pages into whichever page you set as your preferred domain.