You can click update by going to themes or dashboard > update. Changes made using the theme-options or customizer will stay intact after the update but if you have modified the theme core files or added additional codes to theme files then you will lose them with the update. In that case, the solution will be to create a child theme and move the custom code added by you to the child theme.
You can create the child theme using a plugin or manually create one. There are many plugins available. Note creating the child theme at this stage does not move your custom code automatically to the child theme.
To know more
https://developer.wordpress.org/themes/advanced-topics/child-themes/