I'd like to write
\let\newfontinstance=\newfontfamily in fontspec.cfg, but I cannot find that.
May I please know where I can find it?
You should copy the one that already exists in your texmf tree and put it wherever you need it. This is true for all LaTeX .cfg files. Either put it in the same directory as your document (if you only want the cfg file for that document) or put it in your local texmf tree so that it overrides the default one.
You can find out on the command line.
This should give you the full path to this file.