To edit CSS in a Shopify theme, follow these steps:

1. Log in to your Shopify account and go to the Online Store section.

2. Click on the "Actions" button next to the theme you want to edit, and then click "Edit Code".

3. In the code editor, you will see a list of files on the left-hand side. Look for the file called "theme.scss.liquid" and click on it.

4. This file contains all of the theme's CSS code. You can scroll through it to find the CSS code you want to edit.

5. Once you find the CSS code you want to edit, make the necessary changes.

6. Click the "Save" button to save your changes.

7. Check your store to make sure the changes have been applied as expected.

Note: it's important to be careful when editing the CSS code as it can affect the appearance and functionality of your website. Always make a backup copy of the original code before making any changes, and test any changes on a staging site before deploying them to your live store.