People often ask what happens to customizations after a theme update. Well the short answer is: They will get lost.
Why is that? When you update a theme there is normally no build in update routine like you have it for WordPress core updates. That means when a new version of a theme comes out you need to replace the old files with the new ones and that overwrites your custom changes. By the way this also would happen in case of a WordPress update if you would have changed WordPress core files. That is the reason why you should never make any changes to WordPress core files. But how can you customize a theme and stay safe in case of an update? The solution is: A child theme. Read more