Hi @J
Have you changed any core file of the theme? If you have changed core file of the theme those changes will be lost on theme update.
If you have not changed anything but added custom CSS from Appearance >> Customize >> Additional CSS than that CSS will not be affected by theme update.
If you have changed core file, it will be better to keep backup of your files and update theme and replace changed files after complete update.
We recommend you to create child theme and make changes to prevent loss during theme update.
Thank you.