Sorry...you can't. According to the site, "
One-time purchases made between June 1st and December 31st, 2016 include Lifetime Updates. This does not apply to monthly payments." Also, as it says in the lifetime updates FAQ:
Can I still buy a membership version or renewal during this time period?We'll only be selling lifetime updates for SONAR Platinum during this period. It's a better value and will reduce confusion over which version to buy. We want customers to have the best experience. You can still purchase renewals and membership for SONAR Professional or SONAR Artist. Visit our pricing page for more information.