change to cheaper plan

This depends on how much time you have left on your current subscription. If you do not have too much time left, you can to into your Norton Account and disable the Automatic Renewal for your current subscription.  See this Norton Support article.   https://support.norton.com/sp/en/ca/home/current/solutions/kb20090818144...   Then you are free to purchase a less expensive subscription from a reputable retailer and install that product key into your current product when it is due to expire.

If you have just started your new subscription within 60 days, you can request a refund for the more expensive subscription and then purchase the less expensive subscription as noted above.  See this Norton Support articled.   https://support.norton.com/sp/en/ca/home/current/solutions/kb20100113164...