Not sure how to edit your theme? Read this article first.
Find your cart template
Find your cart template in the Templates, Sections or Snippets folder.
You're looking for the file that contains the markup that displays the header of the page (including logo, menu...)
Typically it's called "header.liquid".
Take a backup
Copy the content of the template before you make any changes. Create a separate file and paste the content. Repeat for every file you plan to change. Better safe than sorry!