Favicons on Multisite with Upfront – How to?

I use Spirit Theme on a Multisite and cannot manage to add the site specific favicon.
From WP backend it seems not possible, when clicking on "Appearance / Customize" the Upfront editor launches.
On the Files System in FTP I cannot see a folder for this instance, since it is a Multisite. (Used to place the favicon in root of site…)
Do I need to change the theme itself (uf-spirit) but then the favicon would be attached to the theme and not to the site and with a theme update lost.
Please advise, thank you
Monika