We always take great care in making theme updates, to minimize the chance for something in your content to go wrong. But please always read the instructions for individual updates in the changelog.
Kirby CMS is a flat-file system so every update is done by simple copy-paste, or FTP file overwrite.
KIRBY BEGINNERS NOTICE: Never update content folder. It is the folder where your website content is stored.
Also when updating/overwriting take care and skip overwriting these (if not said otherwise):
config files (..site/config/ folder), because you certainly made some changes to those files
language files (..site/languages/ folder)
Zero One theme update
Zero One is a plugin so you will usually just have to update site/plugins/zero-one/ folder.
If we change something in the assets folder, add some new config option or integrate a new plugin you will be notified about that in the changelog.
When Kirby updates happen you can update it yourself, just update the content in zero-one/kirby/ folder.
If there are some big changes in Kirby we suggest you wait that we first update and test, and if needed make some fixes. You will find detailed update instructions at https://www.one.thezero.club/changelog if that happens.