Unfortunately, that upgrade option was only available for a short time during DMS's initial release. The upgrade option is no longer available.
Therefore, you will need to migrate. In my opinion, the best course of action would be for you to use an application such as MAMP if you're a Mac user or XAMPP if you're a windows user, what these applications do is basically turn your computer into a local server. There you can install WordPress and DMS and then recreate your website locally using DMS functionality and sections. Whilst keeping your site powered by PageLines Framework live.
Once you have completed your site locally using DMS, export all your posts/pages using the built-in WordPress import/export tools and use the DMS import/export options. Then on your live site, install DMS, make sure it is the active theme and then import your DMS theme settings and WordPress posts/pages (if required).
As always, perform a backup before proceeding with the migration.