links are http, but I need them to be https

wp-d-tree links are not secure.
I have a wordpress multisite and am using wp-dtree to provide categories navigation in a help documentation website which is on https and is also contained within another secure site via iframe. I cannot update wordpress environment or plugins at this time. Can someone please advise me how to modify this plugin to force all wp-d-tree category navigation links to route to https instead of http?
I am using the ssl insecure content fixer plugin, but it is not working with w-p-d-tree. Can I edit the plugin files directly? Which file and what is the fix?

The page I need help with: [log in to see the link]