MarketPress plugin for wordpress

Hi, I have "inherited" management of a Wordpress site that uses the MarketPress plugin. The version I have is an older one, and I have been directed to your site to update it, but I can't see how.

Marketpress Version 2.9.6
WPMU DEV Dashboard Version 4.2

This site is in production and I don't want to break anything. All I am trying to do is upgrade to get rid of security flaws which your notes say exist in the older version.

Any advice would be appreciated.

Best, Brian.