Unpacking the package…
Installing the theme…
The package could not be installed. The theme is missing the style.css stylesheet.
Theme installation failed.
Bought a New License after not being able to get our Wordpress site to update the BeTheme, trying to install it now without losing what we already did. This is painful
Please see this short article from the knowledge base, I think it can help: https://help.market.envato.com/hc/en-us/articles/202821510-Theme-is-missing-the-style-css-stylesheet-error
Also, to not loose custom changes you make to a theme, it’s recommended to create a child theme (at least this is something I always do, to make sure I don’t loose anything). There are a couple of plugins you can find to automatically generate a child theme, where you can put the custom changes you make.
Also, before a major update, it’s also recommended to create a a backup first, using the user interface from your web hosting provider (cPanel, for instance, if they have) or using backup plugins.