How do I add current subscription days to new subscription

subscription expires in 16 days...how do I download new subscription, install and not lose the 16 days . I need to be able to add the 16 days to my new subscription

The only way to do this is to contact Norton Support.   www.norton.com/contactcs   Although during the Covid-19 pandemic, these support contacts can be extremely difficult to access. Some have better luck using Twitter  http://twitter.com/nortonsupport

Or you can just wait until one day left and enter the new product key into your current product by clicking on Help > Enter Product Key at the bottom right.