The "code view" button is a "toggle"-style button at the top of the content editor for product descriptions, collection descriptions, page content and blog posts.
The button, which has a "<>" icon on it, is located at the far right of the editor.
Click this button once to enable code view. Click it again to switch back the regular editor view.
Note that there will, in most cases, be more content in the code view that is not visible or is shown differently in the regular view.