There are numerous ways to update the theme.
I suggest you look at this https://xstore.helpscoutdocs.com/article/63-theme-update and in particular update your site by following the “Update using Automatic Updates” section.
It is suggested you backup your site before attempting to update the theme.
You may have a backup plugin already installed. Log into WordPress and check your installed plugins. I have a backup plugin called WP File Manager.
If you don’t have a plugin, then try the below:
https://wordpress.org/plugins/updraftplus/
I’ve used it before and it’s great. I’m unable to find a link to the WP File Manager plugin but it doesn’t matter as the above link should help backup your site.
