How to reduce post preview length on category page

Hi,

I love the theme; however on each of my categories’ pages, my post previews show up as the entire article. I only want the preview to be the picture, the headline, and maybe a few lines from article. Is there something built into the theme that I can use to do change the preview length? Because I do not feel like messing with the code.

site: theamericanyets.com/

Thank you