How do I change archive settings in Twenty Eleven theme

Hello. I’ve recently discovered that my Twenty Eleven theme is displaying my full posts in archives. I’d like to change this to make sure only an excerpt of my posts are showing in archives (I have a paywall up, and the content is bypassing my paywall). I run a Twenty Eleven child theme, so I can make the changes without a problem. What changes do I need to make? Thank you for your time.

Aaron