Use excerpt on post feed?

Hi,

I’m thinking about using this theme for a new personal project I’m working on, but I can’t work out how to change the full text on the blog roll to the excerpt.

https://wordpress.org/support/topic/summaryexcerpt/

This post says I’d need to change the PHP, which it fine. However, there is no reference to the “the_content()” function within the index.php file.

What would I need to change in order to display excerpts instead of full posts? I don’t want to add “Read More” tags, as my excerpts are different from the start of the article.

Thanks,

Kev