So, you have an active subscription for Norton Security which is due to be renewed. In order to replace it with Norton 360 you need to turn OFF the auto-renewal for Norton Security. Then you need to buy Norton 360. After the purchase you can download Norton 360 and start the installation process. The installer will remove your Norton Security first, and then install the new Norton 360 product.
Here is more about the installation procedure for Norton 360.
Please note that the remaining subscription for your Norton Security cannot be combined with Norton 360 subscription. The best option is to use Norton Security until it’s about to expire and then replace it by Norton 360.