Theme update only affect the files in the parent theme folder - you should only update parent theme. If you customized them, these changes will be lost.
Everything else like content, settings, plugins and their settings, child theme files customization will stay.
If you have any problems after an update - please raise a support ticket. Especially this might happen if you skipped many updates. Probably something is changed in the way theme work, which may require settings or content altering.