Theme causing site to fail

I am receiving the following error:

Fatal error: Call to undefined function add_theme_support() in /home/content/11/5214911/html/wp-content/themes/modularity-lite/functions.php on line 26

This was after I tried to update to a new theme on the site. I am really a novice on site-building. Please give basic instructions on how to fix this problem.

Thanks a lot,

Struggling Leader