In late 2017, Shopify updated the "Customize" theme area.
Finding the 'customize' button or link
This area is accessible by following this path in the Shopify admin Sales Channels > Online Store > Themes or clicking here.
To make changes to your current, live theme, locate the indigo "Customize" button as shown here:
You can also make changes to a unpublished theme by finding it on the list under the live theme preview and clicking the "Customize" text link:
Please remember, however, that any changes made to an unpublished theme will only become visible to shoppers when the theme is published.
The 'customize' view: Homepage sections
By default, the "customize" view loads with your store's homepage. The left sidebar gives you access to the sections on the homepage:
Using the page type dropdown
To change settings on different page types (such as a product page or collection page), click the "Home page" dropdown menu. This opens the "page type dropdown."
You can now select the page you'd like to edit.
The sidebar should update to look something like this:
In this example, you can see the "Product" section that, when clicked, lets you make changes to how product pages.
It's important to note that only the homepage and page.details pages are compatible with a large selection of sections. Product pages, collection pages, blog pages and article pages typically only let you modify the sitewide header and footer and main content of the page.
Editing sections-enabled pages
You can also use the page type dropdown to navigate to pages that use advanced layouts such as the contact page or page.details template. Because, in Out of the Sandbox themes, page.details contains the ability to use multiple sections, you will be able to build your page by adding sections and dragging sections around in the left sidebar, similar to how you can build your homepage.
Note that page names generally appear lower in the dropdown menu and maybe require you to scroll down to see them.
Previously known as general settings, the "theme settings" tab lets you control a variety of sitewide settings.
To save your work, be sure to click the "Save" button in the upper right.