1
Duplicate your live theme
In Online Store → Themes, click ⋯ → Duplicate on your current theme. This copy is your safe fallback.
2
Add the new version
Upload the new ZIP, or use Update if Shopify offers it in your theme library.
3
Copy your settings
Shopify carries settings across when it updates a theme for you. If you uploaded the ZIP yourself, set up the sections and theme settings again in the theme editor, using your duplicate as the reference.
4
Preview, then publish
Check the home page, a product, a collection, the cake builder and the cart on desktop and mobile, then publish.
Code changes you or a developer made to theme files are not carried over by an update. Keep a note of them so they can be reapplied.