To change the favicon, you can upload through AbanteCart Cart admin panel (I think you can also upload through FTP by replace the default favicon to yours). The easiest method to do is already explain by llegrand.
If it not change (also happen to me), you have to clear cache (explain by llegrand).
Then hard refresh your page (press control+F5 twice or more)
If it not change (also happen to me), you have to clear cache (explain by llegrand).
Then hard refresh your page (press control+F5 twice or more)