How to only show summaries of posts, instead of the entire post on front page

Hi all,

When people go to https://www.cakeliving.com i want them to see only summaries of all my posts with a ‘read more’-link rather than displaying the entire posts. I have seen another topic on this saying that i need to replace “the_content()” with “the_excerpt ()” in the index.php file, however i can’t find that wording in my index.php file in my twentytwelve child template at all.

Can you help me?

Thanks in advance.