When a new version of your theme is out, you add it to your store as a draft copy of your theme, with your settings and content already in it. You check the copy and publish it when you are ready. Your live store does not change until you publish.
Only themes installed from the Shopify Theme Store get updates. If you or a developer edited the theme's code, save a copy of each changed file, because code edits that conflict with the new version are not carried over.
What the update keeps
The new version keeps these from your current theme:
Theme settings, such as colors and fonts.
Sections and blocks you added, moved, hid or removed, with their settings, images, videos and text.
Templates you created.
App embeds and app block settings.
Wording you changed with Edit default theme content.
Code edits that don't conflict with the new version.
On your theme, click the update notification. It shows in place of the version number when a new version is out.
Optional. Click View release notes to read what changed.
Click Add to draft themes. A copy named "Updated copy of" and your theme's name shows in Draft themes.
If the copy shows "Theme added: code edits could not be included", copy your saved code edits into the new version, or ask the developer who made them.
Check and publish the new version
On the updated copy, click Review.
Check your home page, a product page, a collection page and the cart.
In Draft themes, next to the updated copy, click Publish.
The new version is live as soon as you publish it. Your old theme moves to Draft themes, so you can publish it again if you need to.
Shopify's guide to updating themes also covers automatic updates, which Shopify applies for bug and security fixes without changing your settings or content.
When a new version of your theme is out, you add it to your store as a draft copy of your theme, with your settings and content already in it. You check the copy and publish it when you are ready. Your live store does not change until you publish.
Only themes installed from the Shopify Theme Store get updates. If you or a developer edited the theme's code, save a copy of each changed file, because code edits that conflict with the new version are not carried over.
What the update keeps
The new version keeps these from your current theme:
Add the new version
If the copy shows "Theme added: code edits could not be included", copy your saved code edits into the new version, or ask the developer who made them.
Check and publish the new version
The new version is live as soon as you publish it. Your old theme moves to Draft themes, so you can publish it again if you need to.
Shopify's guide to updating themes also covers automatic updates, which Shopify applies for bug and security fixes without changing your settings or content.