Help updating a Wordpress theme with Envato Market

I’m trying to update my WordPress theme (Valenti Child / Valenti) with Envato Market.
Unfortunately, Envato Market does not seem to see that a previous version of the theme is already installed on the server, and that an upgrade is desired.
So there is only one option: Install.

When I click on it, it download the theme. Then it begins to install the theme, but when it sees that the destination folder already exists, it stops, telling the theme installation has failed.

What can I do?
If I manually upgrade the theme by copying the new files, will it erase all the customizations I made?

Is better to ask author of theme what’s new and which files will be changed.

Also make full backup first.