Hi, I inherited a k2 site, and don't know much about k2.

I made a new folder in my template/html folder called mod_k2_content. In it I placed the default.php file with my alterations. It is not taking.

Previously, I copied the whole module_k2_content folder from the core modules folder to the html folder and updated the tmpl/default.php, but that didn't take either.

What do I need to do to get an override of this file?

Thanks.

You need to add the subtemplate's folder as well.
Eg:

/html/mod_k2_content/default/default.php
/html/mod_k2_content/slideshow/default.php