Hello, Webpat,
Thank you for getting in touch with us.
In case customization was done not in parent theme files or files of plugins, you will not lose it.
You can read about backup here – https://xstore.helpscoutdocs.com/article/63-theme-update or contact your hosting provider and ask them to help you create a full backup.
We hope this information is helpful.
Kind Regards,
8theme team